What are your favorite add-ons for Firefox?

• Reopen closed Tabs

I often want to reopen a tab i recently closed, so this is very handy for me.

• Languagetool

On the fly rule-based Spelling check. Works very good and in many languages. And the best: It's Open Source.

• Facebook-, Google- and Microsoftcontainer

Uses the Firefox Tab-Container Fwature, to lock those companies in Tab-Groups just with themselves. I don't use Tab-Groups aside of that, so it comes in handy.

• Firefox Translate
• (Not really an extention, but still nice) Firefox Gnome Theme, for my personal machines and Firefox UI Fix for the machines at work to make Firefox look more at home on Linux and Windows.