FoxyTunes
Again, a really simple addon, it's only purpose is to save you time tabbing between programs. It adds a little music control bar to the bottom toolbar in firefox. This enables you to do the usual next/previous/pause/play/stop/volume changes to your currently playing music, without leaving firefox. It's compatable with all music players that are worth using, and it's very handy to be able to pause your music instantly if say, the phone rings.

Gspace
This is an addon I only came upon recently, and It's already proved to be a very handy tool. Basically it lets you use those 7 Gigs of storage space wasted on your gmail account to actually be used for storing files. This is very useful, because now you can upload a file, say a word document, movie, picture, whatever it is, to your 'gspace'. You can then access that file from any internet-enabled computer in the world. "Why couldn't I juse e-mail it to myself?" I hear you ask. Well, gspace also provides a friendly user-interface, and facilitates uploading a lot of files at once, wheras gmails upload system requires you to upload each file seperately. Therefore it's not ideal for say, uploading an entire album of photos.
ScrapBook
This should be renamed "The project addon". When you're in the initial phases of looking up stuff for a project, especially something like an essay, this addon is your best friend. It allows you to make what's sort-of like a bookmark album, dedicated to a perticular purpose, like an essay. However, it's usefullness comes from it's ability to bookmark parts of a web-page, and then view all of these parts together on the same page. Hence the name. If you see a reference, you can 'tear' it out of it's web-page, and save it with scrapbook, and then when you're done researching, you should have a nice long page full of references, and their sources, all in one place. Very handy indeed!
StumbleUpon
This is the best "I've got nothing else to do" addon. Boredom is it's mortal enemy. Basically what it does, is put a "stumble" button up the top of your firefox toolbar, and when you click it, it randomly brings you to a page related to your chosen subject (eg 'Humor'). It's a great way of discovering good web-pages you would otherwise have never heard of! It's also great for just killing time!

Tab Mix Plus
This is probably one of the more function-oriented addons, competing with FlashGot. It basically gives the user a huge array of options for customization in firefox, as well as a lot of handy brand new features. For example, it expands the "Open last closed tab" button into "View list of recently closed tabs". It also allows you to add loading bars to each tab, if you are the kind of person who likes to browse through multiple pages simultaneously, this is a great feature! It also allows for auto-refresh of your chosen tabs, which is great for monitoring things like results, or status updates which are posted to web pages. This is the first addon I download on a fresh installation of firefox.
Web Developer
This addon is a great compliment to the FireBug addon. Web developer is a great addon for simulating your website under different conditions, and it has a lot of handy features, for example you can see how your(or any) website would run without javascript. Another great feature of this addon, is that it creates a real-time error log when you are running a webpage. This is infinately usefull if you are creating a web-page, and need to know if there are any errors on it, or conversely, if you know that there's an error, it will tell you exactly where it is, right down to the line of code that the error is contained on. This saves hours of time trudging through faulty code looking for which part is 'broken'.
And that's my list of the best firefox Addons going! Each of the above come highly recommended by me, and I would strongly suggest downloading them you won't know what hit your firefox after the changes!
On a side-note, if you were wondering what theme I use for my firefox, I go for "Noia 2.0". I find it gives firefox a fresh new look.
That's all for now!