2. Adblock Plus
Are you tired of ads? Then you should install Adblock Plus. You can use the extension to remove online advertising and block all known malware domains. It works pretty well.
As you've probably heard by now, if you want to maximize the battery life of your laptop, you should install a Flash-blocking extension of some kind. Flashblock works well for Firefox. This gets rid of those pesky Flash-based ads that populate websites. You can always click on an animation to see it if you really want to .
4. US Spellchecker
This was probably one of the first extensions that I installed. The US Spellchecker will spellcheck your typing inside dialog boxes in your browser windows. If you do any blogging or writing online, this is the way to go. While it's nowhere near as effective as MS Word or dedicated spellcheckers included in word processors, it gets the job done.
While I personally don't use Delicious anymore, nor do I use Pinboard.in, the Firefox extension is really nice to have. Delicious has confirmed that it's currently being sold by Yahoo, so there's no danger of losing all of you bookmarks, but there was a definite Delicious exodus a few weeks ago when we learned that Yahoo was "sunsetting" Delicious. Pinboard saw a heavy spike in users and their price went up from $7 to $9.14.