Firefox has the ability to add many extensions. This web site lists some of the better ones.
This list was compiled in March, so I don’t know which are still good in Firefox 3.0.
Firefox Control and Performance
FasterFox – Tweak settings to make it even speedier.
SessionSaver – Saves sessions and restores browser exactly as it was.
TimeTracker – Tracks online browsing time and pauses when browsing is stopped.
CookieSwap – Manages cookies
IE Tab – Opens a page in Internet Explorer without leaving Firefox.
ScrapBook – Saves webpages and organizes your collection properly.
ColorfulTabs – Gives each tab a different color.
PermaTabs – Creates permanent tabs that can’t be closed and will stay in future sessions.
Web Development
Web Developer – Adds a menu and toolbar with various web development tools.
Firebug – Integrates with Firefox to put several development tools at your fingertips.
MeasureIt – Draw out a ruler to get the pixel width and height.
Image Zoom – Easily zoom in, zoom out, or set custom zoom options for any image.
ReloadEvery – Reloads webpages opened in a certain amount of minutes or seconds.
LinkChecker – Checks the validity of any links on a webpage.
Blogging and Social
ScribeFire – A blog editor that integrates with Firefox to make psting easy.
Sage – A lightweight RSS and Atom feed aggregator.
Adsense Notifier – Displays Adsense earnings on the statusbar.
Shareaholic – Shares bookmark, and email web content without leaving the page. Includes Digg, StumbleUpon, Del.icio.us, Mixx, Facebook, and more.
StumbleUpon – The StumbleUpon toolbar for Firefox. Discovers pages for you based on your interests and what you like and don’t like.
TwitterFox – Notifies you of your friends’ Twitter statuses by adding a small space in the status bar. Also lets you update your own status.
Miscellaneous
Remove It Permanently – Removes annoying web content and hides it forever.
PictureThis – Pulls up pictures from Google Images of any word or words highlighted, to help understand the text.
Clipmarks – Instead of bookmarking a whole page, selects and clips parts of the page.
MyStickies – Places colored sticky notes all over the web and organize them with tags.
Forecastfox – Lets you check the forecast easily without ever leaving your browser.
See these and more here.
These undoubtedly work on the Windows version. I don’t know about the Mac version of Firefox.
