So that I can reference what I use in future posts, I've already written a list of security and privacy extensions I use with Mozilla Firefox and commented that they're the primary reasons I choose to use it over every other browser. Using a browser without those extensions (or equivalents) makes me feel naked and scared now! You can take that however you want.
I've installed far too many extensions over the years that I've later never used, but these are the usability ones that have stuck.
Absolutely essential, it reduces irritating distractions on pages and makes them render much faster because you're downloading less stuff. Some people complain about Adblock Plus. I ask how many of them own a DVR which skips ads, or how many of them have a WC break or a snack instead of watching sponsors of a sports event. Enough said.
Greasemonkey ets you modify the appearance and function of individual sites and pages by clicking and automagically installing scripts from sites such as UserScripts and UserStyles. Once you have scripts installed, they work transparently and so fast you can't tell they're even there.
I started using this when I thought Google messed up Reader's interface.
I mostly use the shell to quickly get sftp work done, but for tedious and repetitive tasks FireFTP makes it really simple. If you're one of the few who haven't tried it, it launches itself as a tab within Firefox and uses the split file manager metaphor like Norton Commander with the left pane showing your local drive and the right showing the remote server.
FireFTP is honestly polished enough to be a standalone application.
I find it curious that even if you explicitly download the "English (GB)" versions of Firefox, it still comes with the American spelling dictionary which battling with gets pretty old fast.
This extension is fairly old as far as extensions go, but it still works even in the lastest Firefox 3 builds. And besides, you could argue spelling doesn't change as fast as builds of an open source project do. Wait, ignore what I just typed; that's a Pandora's box I would rather leave shut!
If you don't like being scolded for not peppering your words with Zed, grab it!
Adds a world clock to either your status bar or your bookmarks bar; I choose the latter.Insanely useful to have right in your browser where you're most likely to be viewing things and conversing with people from other parts of the world.
For example, I can check just by looking at it what the time is in Singapore when I'm in Adelaide and vica versa, as well as the eastern Aussie states, the UK, Talkeetna, Toronto, Tokyo... I thought that was some clever alliteration.
The Life-of-request info extension adds a really useful monitor to your status bar that shows how many seconds, how much data and how many requests a page took to render. Supposed to be used for people who want to monitor the performance of their own websites, but I use it for everything!
For example, did you know a Whole Wheat Radio page heavy with images and other media renders faster than many commercial websites? Wonder if it's faster than the RIAA or ARIA...
I discussed these late last year. In a nutshell, these themes make Firefox look much more Mac like. Firefox 3 was a huge improvement over version 2, but it still looks a bit kludgy.
I use this theme for Firefox on my FreeBSD and Linux boxes. It replaces the default toolbar icons and styles with ones that match your other GTK+ applications. Fits really well with Xfce and window managers such as dwm or OpenBox where you're using mostly GTK+ apps like RoxFiler and Gnumeric.