In one of my comments I mentioned that I use a lot of different extensions for Firefox. But why and what are they? Here I'll share the ones I use and give a short description on what they do.
Search engines [To add click on "Add Engines" in search box drop down menu]:
-- gahooyoogle - This search engine shows google and yahoo search results side by side so that you can access them both at once.
- Forecastfox - Shows weather right in the browser [can have several locations].
- AdBlock - Blocks annoying ads [has a default filter but you can add by right clicking on an ad and setting to block it].
- Adblock Filterset.G Updater - A nice updater for adblocks filer list.
- Flashblock - Blocks all flash from playing and allows you to manually tell what to play [Still doesn't work with Firefox 1.5 :(].
- Gmail Notifier - Needles to say more.
- ColorZilla - Must have for web designers, view RGB values of colors in your browser window with one click.
- Sage - RSS aggregator, for those who are lazy to use bloglines [like me].
- StumbleUpon - Stumble on different pages suggested by other users. I found a lot of very cool and useful sites [Warning: can get addictive].
- Download Statusbar - Bring your downloads to your statusbar.
- GooglePreview - Allows you to see preview of webpages in Google results.
- OpenDownload - Allows you to choose whether you want to let the OS open the downloaded file [Not supported in Firefox 1.5 yet :(].
- CustomizeGoogle - Let's you customize google's search results page [delete ads and etc].
- Disable Targets for Downloads - Doesn't open extra tab/window when downloading zip/jpeg/mp3 and etc files.
- Mouse Gestures - Allows you to use mouse gestures to controll your navigation through the web [back/next/reload/go-to-homepage and etc].
- FlashGot - Allows you to download files with external programs like FlashGet/ReGet/GetRight and etc.
- IE View - Allows you to launch IE with current page [if it's not supported but you really want to see the page].
- HashColouredTabs - Colors tabs with no icon in different colors based on domain [ex.: google - red, yahoo - blue].
- Dictionary Tooltip - Double click/Ctrl+Double Click on a word and see it's meaning from a dictionary.
- Tab Mix Plus - One of the best tabs extentions [even supports restoring closed tabs].
- FasterFox - Speed up your Firefox so it flies through pretty much any page.
- PDF Download - Allows you a choice between downloading PDF or viewing it in a browser.
Note: Post in the comments if you're using other extensions (I focused on those supported by Mozilla Foundation).