I tried accessing Gmail through two different builds of Firefox in a few different operating systems that I have access (including 3.5.2 on a Windows XP system) to or on my own systems and was able to get into it through each time--however, I did notice (for reasons I plan on looking into...) that it seemed to take a bit longer to load on the Windows system that was running 3.5.2 for some odd reason--but it did load within a few seconds (albeit with a progress bar). In contrast, on my Linux system that's still running 3.0.13, it loaded instantly.
By any chance, do you have the Skype add-on installed for Firefox? The reason I ask is because it's been linked to a number of problems tied into AJAX (which Google uses for Gmail) which has been preventing people from getting into sites which make use of it (for things including logins--Facebook is another site where users have been having these problems), amongst other issues.
Another add-on that has ironically been interfering with Gmail was released by Google, themselves (Google Gears) and Microsoft's .Net Framework Assistant is yet another that has been found to be causing problems (in the case of the latter, just disabling it may help) with Gmail.
Also, have you tried getting in through the "plain HTML" version of Gmail?
In the meantime, I have gone through the official bug database and there was a report regarding a problem someone was having of this nature--however, while there was a developer assigned to the report, the person who filed it never followed up on it (even after the developer attempted to contact them regarding it).
In terms of bookmarks, if it's something you recently pulled up or bookmarked, there is an option to just visit recent bookmarks (there is a similar idea with frequently visited sites--however, in the latter, it will require set up by going into the organize bookmarks option--just as it's possible to attempt to search bookmarks manually through the "organize bookmarks" option). You can also attach "tags" to any bookmarks so only those containing the tags get pulled through the search.
However, I admittedly have not really fully explored many of the current bookmark add-ons enough to where I feel I can recommend anything.
As far as plugins go, I've generally not been into "frilly" or "eye candy"-type of plugins or add-ons of any type and have tried to keep things to a minimum and only install things I actually would use.
Up until fairly recently, I would have recommended Adblock on its own as I prefer the customization options on it and ability to completely block out Flash if I wanted to or sort through things and determine what to block versus what to permit and customize filters.
However, it appears (on its own, at least) Adblock is no longer available. Instead, its larger and more popular sibling, Adblock Plus (which will prompt you as to which lists to subscribe to for ads to block upon first loading it) has taken over completely. A number of people out there actually prefer this version as it automatically blocks out ads and supposedly determines ad from genuine Flash application (such as something on Youtube) and therefore they don't have to deal with it.
In having tried it, though, I admittedly didn't care much for it as I felt it was too automated and didn't allow for the same level of control and customization as the standalone version of it allowed for--but then again, I also tend to prefer doing certain types of things manually anyway.
Likewise, another add-on I've been using (which I'll need to actually locate the name of--I'm presently on a different system from the one I normally use where I have this installed, so therefore do not have it on here) displays a "preview" of a page after it's loaded (which I find useful in cases where I may have multiple tabs open at a given time and want to mouse over it to see what it is). The downside to this plugin is also something I've found to be something of an annoyance of where the preview option remains even after I've either exited from the tab or just for a few seconds more until I click on something else (unfortunately, I have yet to locate a comparable plugin of this nature that functions in much the same way).
Aside from that, there are some other plugins I have and use, but would need to get into the other system to actually get the names of.
However, I would advise avoiding the No Script plugin--there has been some controversy regarding the project as well as just that plugin where the developers of No Script were illegally altering certain other add-ons (including anything pertaining to ad blocking (notably Adblock Plus being amongst the victims of this) or blocking of scripts), as No Script actually derives both revenue as well as support through the sale of ads--they were altering the other plugins which were blocking their ad partners to allow them instead. They did make some of the requested changes, but the add-on still remains questionable.
Beyond the drama, though, when I had tried using it in the past (prior to the recent drama), it sometimes would block things that should not have been blocked or caused problems with certain sites and proved to be enough of a headache that I ended up removing it.
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Update:
I just checked and, in case you may have been interested, the add-on/extension I have which allows for the tab previews is called "Tab Preview"--however, one particular annoyance I have with it (as mentioned earlier), is that the previews usually remain there for longer than they realistically should or sometimes won't go away with a simple "mouseover" or will remain there even once the tab has been closed for a bit longer until/unless something else is loaded. The other issue I have with it (although, this is really more a personal pet peeve I have with it) is that it doesn't display the path (url/address) upon doing a mouseover of it either. Then again, I think I was a bit spoiled by having had such a feature in Firefox's sibling browser, Sea Monkey where it's built in (albeit, was implemented by Sea Monkey's own devs as opposed to the situation with this plugin and Firefox, where it was an outside developer who submitted it).
In having had another look at my other extensions and plugins that I rely upon for the most part, I also came to realize just how many of them may/may not be available in other platforms.
