Below I'll make some broad observations about the publication date progression of the F&GM stories, which is not the same as their ultimate in-world continuity (publication history taken from Wikipedia). Warning: There are SPOILERS liberally thrown around below.
1940s – A half-dozen stories of disconnected roguish adventures of Fafhrd & the Gray Mouser. Half of these have page counts in the teens, including the memorable "The Howling Tower". Two are around 30 pages, including the excellent first publication, "Jewels in the Forest" (1939). Later in 1947 the 90-page novella "Adept's Gambit" was published in Night's Black Agents, actually the very first thing written about the pair by Lieber, which is unique in that it actually sets the heroes in classical Earth (based in Tyre). Most of these were collected in Swords Against Death (1970), the 2nd book according to in-world continuity.
1950s – Only 4 stories were written in this period. Two are the 26-page stories "Claws from the Night" (1951) and "Seven Black Priests" (1953). In 1957 a collection of the stories to this point was released called Two Sought Adventure (geez, compare the figures on that early cover!), including a new 2-page "Induction" as an opener. In 1959 the 41-page novelette "Lean Times in Lankhmar" appeared in Fantastic, with its heroes archly opposing each other, great detail on the worship of the gods in Lankhmar and the gods of Lankhmar, and the Second Coming of Issek of the Jug. (Again the earlier short stories appear in the 2nd-by-time book, Swords Against Death.)
1960s – About a dozen stories of various length appeared in this period (more than in the prior 20 years combined). This includes fan-favorite short stories such as "When the Sea-King's Away" (1960), "Bazaar of the Bizarre" (1963), as well as "Unholy Grail" (1962) which tells the apprenticeship of the Gray Mouser. In 1964 the 94-page "Lords of Quarmall" was published in Fantastic -- my girlfriend & I both found this to be rather dreadful reading, and at the end of the first Gollancz collection, it had me worried about the later works (but it turns out, this was actually among the very first pieces of writing, started by Lieber's partner Harry Otto Fischer in 1936 and finished by Lieber himself almost three decades later). Also in 1965 Fantastic published "Stardock", a 65-page novelette, which is a slower-paced work, but nonetheless one of my favorites because of its careful, detailed, and properly arduous depiction of Fafhrd & the Gray Mouser climbing the tallest peak in the world. Near the close of the decade, Lieber published the 230-page novel Swords of Lankhmar, with its city political machinations and the Mouser shrunk down to mouse-size among the denizens of Lower Lankhmar -- again slower-paced, but I found it to be fairly enjoyable when I read it at the start of the 2nd Gollancz collection (probably the first part better, published as a standalone story "Scylla's Daughter" in 1961; the full novel is set in time as the 5th book of the traditional set). The other stories are collected in various books #1-4 of the series.
1970s – Now, it was around 1970 that Lieber's works started to be published as a large collection of books for the first time. This includes Swords in the Mist, Swords Against Wizardry, and Swords of Lankhmar in 1968, and then Swords and Deviltry and Swords Against Death in 1970. At this time we get the current in-continuity ordering of the stories (the 1970 books are #1 and 2, while the 1968 books are #3, 4, and 5), and numerous new short stories to serve as fill-in pieces and rationalize the whole series. In 1970 Lieber published the 74-page novella "The Snow Women", providing Fafhrd's early backstory in the North; as well as the 69-page novella "Ill-Met in Lankhmar" which shows how the pair initially hooked up. (While this latter won both the Hugo and Nebula awards, I find it to be rather strangely toned, unusually tragic and melodramatic.) Follow-up short stories from 1970 regarding this early period are "The Circle Curse" and "The Price of Pain-Ease". These relatively late-written stories appear in Swords and Deviltry and Swords Against Death (books #1 and 2 in continuity). Another half-dozen short stories appear from 1973-1975, ending with 1976's novelette "The Frost Monstreme", and 1977's 98-page novella "Rime Isle" (all of the preceding collected in book #6, Swords and Ice Magic from 1977). Finally, the short story "Sea Magic" was published in the pages of The Dragon magazine in 1977 (and collected with the stories below).
1980s – Lieber published three more, longer works in the 1980s. These are: "The Mer She" (33-page novelette, 1983), "The Curse of the Smalls and the Stars" (49-page novella, 1983), and "The Mouser Goes Below" (177-page novella, 1988). I find these latter stories to be confusing, the plots almost totally inscrutable, at turns unexpectedly porn-y, and just really difficult to follow or remember. All of these were collected in the 1988 book The Knight and Knave of Swords (#7 in continuity).
So on close analysis, part of my normal thesis about pulp-fiction doesn't actually hold up -- it's not so much that the stories uniformly turn longer-form over time and get more bloated (the median page count actually stays at an identical 26 pages through the decades of the 1940s, 1950s, and 1960s, dips to 15 in the 1970s, and then rises to 49 only in the 1980s). It's more that from time to time throughout the years, Lieber tried his hand at longer-form stories, and regardless of the publishing era, it is these stories that I found to be more plodding, challenging, and hard to remember afterward (including the very first pieces of written work "Adept's Gambit" published in 1947, "Lords of Quarmall" published in 1964, along with the later works "Rime Isle" and "Mouser Goes Below" written in the 70's and 80's).
Let's set aside the fact that, to a certain degree, Lieber went down the "re-order continuity and write some origin prequels" path (for a far more outrageous example: see Moorcock). There are really two major tonal changes that occur in the stories of the mid-1970's that gave me the feeling of the series precipitously falling out of its chair. These are the following:
On-Stage Appearance of the Gods. Prior to 1975, the gods never made physical appearance in the Lankhmar stories. While there are a legion of priests swarming the streets of the city and making a riotous noise to the gods in Lankhmar (detailed in stories such as "Lean Times in Lankhmar"), not a single one of them has magical powers, and to my reading they are clearly amusing showmen, charlatans, and frauds. Famously, Fafhrd is mistaken for the god Issek after being shaved, going on a drunken rampage, and miraculously shaking off a series of attackers' weapons (having been sabotaged in advance by the Mouser). If incidents such as this serve as the basis for the other of Lankhmar's various gods, then clearly the situation is not good for them -- and that's very much in the tradition of other works of pulp fiction being generally skeptical or dismissive of the nature of the church or religious men. The gods of early Fafhrd & the Gray Mouser are ambiguous in stature, quite probably just the product of con-men and/or delusions.
A prime example is in the first-published story, "Jewels in the Forest". A holy man follows F&GM into a dungeon-like structure, waving protective signs in the air. He declares: "For forty years I have lived on crusts and water, devoting my spirit to the Great God... My purpose in coming here is to destroy an evil thing... no harm can befall me. The hand of the Great God is poised above me, ready to ward off any danger that may threaten his faithful servant...!" -- and exactly one page later he is, ironically and unceremoniously, very dead. (As an aside, this 1939 story is otherwise fantastic and possibly the most thoroughly D&D-ish piece of writing that I've ever encountered.)
But: This changes dramatically with the 1975 short story "Under the Thumbs of the Gods". I was really rather shocked when the first page of this story introduces the "Land of the Gods" and a host of inhabitant deities bumping into each other and interacting, in particular the trio Kos, Issek, and Mog. The first two represent entities that Fafhrd had served or at least sworn idle oaths to in the past. The third is a spider-god with a rather ridiculous retcon of a back-story in that the Mouser had worshiped an idol of it to get in the graces of his first love -- ridiculous because in all the prior literature Mouser had been fervently and even exasperatingly skeptic and atheist. From this point forward, the gods routinely meddle in their affairs and curse them with a number of distracting ailments (up to and including permanent disfigurement). I was just really surprised by this total reversal in theme.
Settling on Rime Isle. The 1976 novelette "The Frost Monstreme" opens with the pair in their customary tavern:
Fafhrd shook his head morosely. 'We've never really lived. We've not owned land. We've not led men.'
'Fafhrd, you're gloomy-drunk!' the Mouser chortled. 'Would you be a farmer? Have you forgot a captain is the prisoner of his command? Here, drink yourself sober, or at least glad.'
The Northerner let his cup be refilled from two jars, but did not change his mood. Staring unhappily, he continued, 'We've neither homes nor wives.'
'Fafhrd, you need a wench!'
Now, when I first read this, I laughed heartily, thinking it to be a very clever anticipation of a classic Seinfeld scene:
Of course, after George & Jerry commit to getting married in a season cliffhanger, by the start of the next season, Jerry has abruptly broken it off with his carbon-copy fiance and returned to being much happier. (For George the problem lingers for a while longer.) So I was guffawing at how obviously identical this outcome would be for the pulp heroes in their analogous scene.
Except that Leiber wasn't really kidding with this exchange. Fafhrd & the Gray Mouser really do proceed within the story to assemble companies of men for them to gravely captain, to enter committed wife-like arrangements with two new women, and to land in permanent homes in the distant and ascetic community of Rime Isle. They will not ever again return to Lankhmar, or the Silver Eel tavern, or see their wizardly mentors Sheelba or Ningauble ever again in any of Leiber's stories.
In accordance with this shift of setting, the themes and tones of the narrative likewise shifts in a fairly dramatic fashion. We switch from Fafhrd & the Gray Mouser being wandering, thieving, active protagonists in the sword-and-sorcery picaresque style, to more responsible, reactionary, defend-the-land-from-invasions heroes, in the high fantasy style. In so doing, they become quite a bit less interesting, and simultaneously become mostly spectators, often saved by some outside phenomenon of which they are simply fortunate to be in the vicinity. To be charitable, we might say that this resembles the D&D tradition of high-level heroes switching to feudal rulership -- which either speaks poorly of its prospects for gaming, or emphasizes that the PCs must still remain the primary actors and not simply be blown around by setting events.
So I would say: If you want to experience Fafhrd & the Gray Mouser as they are usually acclaimed, in their wandering roguish action-packed short stories, stick to the earlier works in the 1st Gollancz collection (books #1-3 in the traditional series, which are generally excellent!), and you can pretty much skip the 2nd collection without missing much of the good stuff (possibly excepting book #4, the novel Swords of Lankhmar). I guess I would have been happy to read those earlier books and left things at that.