Lets see the first one is DownThemall, I'm not sure if someone already talked about this one or not, but it basically allows you to download what ever you want at a level 1 bases. It comes in handy if there is a lot of stuff on one web page you want whether it be videos to pictures to music, it can download all of them for you without you doing it one at a time.
Next would be Firedownload which makes your download speed much much faster. It has a few glitches in it, but overall it's pretty good, and quite a bit faster when downloading large files. It's supposed to triple your download speed, I'm not so sure about that, but it does seem to be a little faster all the same.
Next would be smart bookmarks bar which allows you to get rid of all those nasty words next to your tool bar bookmarks, so it's just icons so you can have a considerable more bookmarks up there which makes it much tidy and handy. Plus a lot more room on it than before. Before you could have like 5 or 6 I think and now you can have like 25 or something, haven't hit the end yet so don't really know, but you can have a lot more bookmarks on your tool-bar simply because their just icons, no words.
Next would be United States English Dictionary which adds more words to your spell check dictionary so it recognizes more of your words, very hand for bad spellers like me.
And I was going to show you one more, but I can't find the name of it. It is a Japanese add-on on, so I can't find the name of it. It has an English name, but I can't find it so yeah sorry about that. But what it did was put foxy in your download manager and other screens that are normally boring he gets in and makes them interesting. Maybe one of y'all know what the name of this add-on it and can tell everyone, but I can't find the name right now, so sorry.
Next is an add-on called Favicon Picker which allows you to make your own icons for websites, which comes in handy when if you use the add-on I spoke of earlier with turning your bookmark tool bar to just icons no words. So this way if a website doesn't have an icon for itself you can easily make an icon for it with just a few clicks. Very handy.
Next would be FoxLingo which is able to translate any web page, into English or what ever your default language is, it's obviously not perfect, but with some common since, it gets the job done, helpful when looking at foreign sites.
Finally I would say Personas For Fire fox, I love this add-on, I've even made my own persona so, I love it. For those who don't know about it even though so many have already posted on this add-on, it makes your top and bottom browser bars different,with custom images and such, I love it. Check out the one I made and see what you think.
http://www.getpersonas.com/persona/30813
I'm even thinking about making a 2kinds one next, but we will see how that goes, no promises.