Well, this may come as a surprise, but i don't use any.
I like things to be vanilla.
However, I have recently considered using a few.
I do not mind ads, but speaking flash ads make me want to kill someone. Slowly and painfully. So I may consider adblock, which blocks almost all ads.
Then there are a few which would be handy for web development.
But I am still resisting.
How about you lot?
Any extentions you can't live without?
I use firefox like 98% of the time and the only extension i have or use are for the additional search engines on the top right hand corner... which allows me to search ebay without going to their website or search youtube without going to theirs. Other than that im happy with what i have.
Back in BLACK
I also use Firefox 99% of the time.
I only use three:
[url=https://addons.mozilla.org/firefox/1865/]Adblock Plus[/url] as you know blocks most ads.
[url=https://addons.mozilla.org/firefox/3366/]British English Dictionary[/url] checks your spelling as you type.
The search engine extension which Seraph has mentioned.
I have:
All Customized Links (sometimes useful, a reduced favourites list in the right-click menu)
British English Dictionary (good for automatic spell-check)
Cooliris Previews (lets me hover over links for a preview)
DOM Inspector (not sure why)
Download Statusbar (fairly useful)
DownloadHelper (great when it works, de-embeds videos)
easyGestures (hardly used but I like it)
ErrorZilla (options for a 404, such as Wayback version, sometimes useful)
FavLoc (lets me pick a download location; maybe unused)
FootieFox (Football scores in the statusbar; great but not comprehensive eg no FA Cup scores)
Scroll Search Engines (quite useful, pick a search engine with the mouse wheel)
StumbleUpon (Cool sites when I'm bored)
TinyMenu (Tidies up the toolbar clutter into a dropdown, very customisable)
UnPlug (de-embeds videos)
URL Link (for navigation to non-hyperlinked urls)
VideoDownloader (de-embeds videos)
WOT (rates websites for safe browsing)
And a couple of minimalist themes.
I could live without them, some of them are really cool.
[size=10]The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts.[/size]
[size=9]Bertrand Russell (1872 - 1970)[/size]
I just downloaded:
[url=https://addons.mozilla.org/en-US/firefox/addon/4258]Tab Effect[/url] as the name says gives an effect while changing tabs, doesn't make much difference but looks really cool.
[url=https://addons.mozilla.org/en-US/firefox/addon/1320]Gmail Manager[/url] which is way better than Gmail Notifier, and can support multiple Gmail accounts.
[url=https://addons.mozilla.org/en-US/firefox/addon/2207]Cooliris Previews[/url] as Honey's mentioned.
[url=http://www.siteadvisor.com/]McAfee SiteAdvisor[/url] which you may already know works using colour coding (green = safe, yellow = be careful, red = stay away). There's extensions for Firefox and IE.
There's two FireFox extensions for prayer timetables, which I haven't tried yet.
[url]https://addons.mozilla.org/en-US/firefox/addon/4270[/url]
[url]https://addons.mozilla.org/en-US/firefox/addon/3393[/url]