In an earlier poll I was a bit surprised to learn that FireFox was so popular, over 95% have voted for FireFox to date.
So, let's take a little time and please list your favorite, most useful, FireFox add-ons (with links to the add-on) and add a few words on how often and how useful do you find the add-on.
Thanks.
The "end game" will be an table of our recommended FireFox plugs, after a short voting period where we rank the "contestants" 
OK,
here is my list:
I try (enable and disable) a number of add-ons for FF3. Here are current my "must haves"....
I have a few more active, but I could get by without them.
I try to keep add-ons at an absolute minimum. I may be losing out on some of the niftier features, but my time is too valuable for me to try out add-ons just to discover they were written in five minutes by some beginner /-:
Open in Browser - add "open in browser" to that pesky "download" or "open with selected application" dialog. Managed to get this to work in Firefox 3 with a bit of patchwork, although I think some of the recent FF3 upgrades broke that again.
It's All Text - add an Edit button which invokes your favorite editor (Emacs, natch) to edit text instead of those nasty edit boxes which are always too small and don't do anything useful or fun when you press Esc Meta Alt Control Shift
Restart Firefox - incredibly lame that this is missing from Firefox itself. Another one I managed to hand-patch in order to make it work with FF3, but again, alas, looks like I will have to do it again because it's missing from my menu once more.
A couple of runners-up which I use intermittently:
NoScript - a nice safety net, especially if you are confined to a Windows box on a gloomy, desperate day. Gets in the way sometimes but hey, that's security.
Linkwad - pretty lame, but I have not yet found a session manager which would allow me to save sets of tabs in a convenient form, and access them from anywhere. I really need to find something better. I'm a certified tab-aholic so my needs are probably a bit out of the ordinary. Any suggestions for what to try next?
PrefBar - to quickly toggle certain navigator preferences and perform certain utility functions: Colors, Images, JS, Flash, Kill Flash, Animation, Clear Cache, Change UserAgent (UA) spec - IE, Firefox, Moz etc...
I use very few extensions: Mouse gestures (because I'm running a fairly old version of firefox), Flashblock (because I hate flash ads but still want to view some flash contents of my choosing) and OOK to snatch embedded video.
All available from the firefox extensions pages.
Few of my favourites are:
## I recomment below two to everyone using FF
NoScript
McAfee SiteAdvisor
## below two who frequenlty use FF & IE
FireFox View
IE View Lite
## Others
Colorful tabs
Undo Closed Tabs
Table Tools
## For web developers
CSS Viewer
FireBug
HTML Validator
Web Developer
Undo Close Tab is built in in Firefox 3. Perhaps it would make sense to post which version of Firefox you are using?
Let's not get into debating the opinions of other's favorites; let's simply let folks post their favorite add-ons.
Also, please post the links as requested (see example posting formats from era, vgersh99, neo, radoulov). This takes very little time on the part of the original poster; but saves lots of (collective) Google and search time for readers who might want to take a look.
Sorry, my intention was not to question, just to note what I perceived to be a missing minor requirement. Just like the link, I think the Firefox version is an useful thing to include. Many plug-ins which are popular with FF2 are not available for FF3 yet and some are obsolete because FF3 provides the same functionality out of the box. By the time future readers find this thread in Google, FF2 is likely to be mainly historical (although I guess there are still people out there using even FF1).
Good idea. Please post if you are not using FF3, otherwise we will assume you are using FF3.
(BTW, stats for this site show that most users use FF3 and that FireFox use skyrocketed with FF3 was released.)
Adding to my earlier post, I also like these handy add-ons ....
Your link says FireFTP but the link and decription seem to belong to NoScript (-:
FireFTP - I found it here, I think Neo has posted about it; it's excellent;
Adblock Plus - great filter;
Download StatusBar - 0.9.6.3;
MaxOSX Theme 0.6.3 - if it counts - makes Firefox looks metallic, just like Safari, but in some way reminds me of Mercedes AMD CLK63 Black Edition, I don't know why 
Fixed......
I have 2 of the add-ons on Neo's list. When I installed Firefox 3, I found that DOM inspector ceased to exist, and must be installed as an add-on. So, I will list it here.
I didn't see this addon before, so here it is - FlashBlock - "Flashblock is an extension for the Mozilla, Firefox, and Netscape browsers that takes a pessimistic approach to dealing with Macromedia Flash content on a webpage and blocks ALL Flash content from loading." My personal favorite, saves lots of time when loading pages.
Great extension, and you can tell it to allow flash from certain sites that you routinly view flash on too!