Open in Browser: Because some sites insist that I should save that PDF to open it.
Session Manager: a.k.a. "task freezer". Save and restore any or all of your open windows and tabs along with their histories.
Google and TinEye Reverse Image search. TinEye's matching engine is more powerful; Google has a much bigger database.
Offline QR code Generator: the easiest way to send page/image/link URLs and arbitrary text to my phone: [highlight text if applicable,] right click on page/image/link, "Show QR", aim phone.
Restartless Restart: Because it's Firefox and Control-Alt-R is much faster than killing the process.
Context Search X: highlight, right click, "Search with", pick any of my engines. Very flexible; allows custom accelerator keys.
Context Highlight: highlight multiple words or phrases all over the current page. Not perfect but really useful.
Live HTTP headers: Disabled since Fx ships with devtools.
It's All Text!: Edit those pesky textareas in your preferred editor. Perfect for HTML boards and wikis.
And obviously Adblock Plus.
Not shown: custom search engines for Google Images, Wiktionary, Google Translate, Gmail...