Apart from what everyone already posted:
- Boring RSS - displays an rss icon in address bar with the rss feeds from the current page’s head tag - the cool thing is that unlike other addons like this, this one has only the activeTab permission, rather than “access your data for all sites” - https://addons.mozilla.org/pl/firefox/addon/boring-rss
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uBlacklist - hides some pages from search engine search results (I use it to hide reddit) - https://github.com/iorate/ublacklist
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Tridactyl - like Surfingkeys, Vimium etc. - more vimlike experience for Firefox - you can also optionally install a native extension to run shell commands in the os from within ff (yeah dangerous): https://github.com/tridactyl/tridactyl
Did you see the new ff vertical tabs in nightly firefox labs?