These were all from back in the FF3 days or so, circa 2010. Most of them are still around, which sort of says something. See my Essentials collection for a more recent vintage.
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on...
Control what gets sent as the HTTP Referer on a per-site basis.
Show the IP address of the current page in the status bar. It also allows querying custom information services by IP (right click) and see location and avaiable vouchers for shopping sites. Additionally you can copy the IP address to the clipboard.
FfChrome is a nifty little addon that slims and trims your context menu on the fly for simplicity, ease of use, easy access and eliminates the clutter. One addon that makes your day-to-day life easier by increasing productivity.