Along the lines of the similarly popular "Utilities, Share Yours" thread.
What Firefox extensions do you use? (This thread is also appropriate for themes and other Firefawks-related stuff.)
Be kind, link to them via this site:
If they're not on addons.mozilla for some weird reason, link to a site where you can download it.
https://addons.mozilla.org/en-US/firefox
NoScript
Every person who uses Firefox should use this. It prevents any scripts or Flash from auto-executing unless you explicitly allow it.
Adblock Plus
Blocks Ads and better than Adblock IMO. You used to need Filterset.G to go with this, but not really anymore as Adblock Plus has filters built in.
ForecastBar Enhanced
Enhanced version of ForecastFox. Highly customizable. I love being able to look at animated radar because I can estimate roughly how much time I have before a storm hits. Great for general use but supreme fun for weather geeks.
ColorZilla
ColorZilla basically lets you figure out what a color is on a page. Click the lil' ColorZilla icon and then hover over a color, and it gives you a RHB
and hex value for it. Great for web developers, artists, etc.
MeasureIt
Click the icon and drag a box that tells you the size of stuff in pixels. I mostly use it for making websites, .html files, etc.
ChatZilla
IRC in Firefox. Sadly this runs in a seperate windows instead of as a tab.
If someone knows of an extension where I can run IRC in a tab within Firefox, I'd love to hear about it!