I was just checking the extensions installed on my Firefox and got a pretty big list:
Adblock to remove unwanted ads;
Greasemonkey to change pages (although Slashdot is the only one right now);
Firesomething, just for fun;
Deepest Sender to post things on LiveJournal (yeah, one extension to post on something you can do over the web);
SpellBound to spell check stuff on Deepest Sender and on web forms;
FoxyTunes to control my player on Windows and allow Deepest Sender to get the current song.
Quite impressive, if you remember that Firefox is just a browser. Well, maybe Firefox/Mozilla is the Emacs of web browser, then.
Edit 1: Ok, two more, just for the fun of installing more:
ForecastFox for weather information (quite cool that it shows the current temperature and the forecast for two days), and
Gmail Notifier to check my Gmail account.
And I won’t even mention the amount of smart bookmarks I have…