Need to bookmark this so I can find it again…
QuickTabPrefToggle
Adds some great preferences/options relating to tabs, etc. (for example, can force links that would ordinarily open in a new window in a tab in the background.
The author of that extension also has a few others too that look interesting. Here’s the extensions homepage
Usually when I’m discussing computers with my parents, it’s always me telling them about one thing or another. But since I don’t use Macs that often, my mother showed ME something I didn’t know about Mac IE (I was pleading with her to use another browser). The one feature she was reluctant to give up was something called “scrapbook” - which I had never heard of. Not exactly like a bookmark - in that it saves a copy of the page to your local machine. Good for if you have a “reciept” for a purchase online, and you want to “save it.” I was SURE there had to be an extension like it for Firefox - sure enough, there is:
ScrapBook :: Mozilla Firefox Extension
(And now, thankfully, the world has one less IE user on the Mac) 
Just spotted this on the del.icio.us news blog:
Foxylicious
Import your del.icio.us bookmarks into firefox.
Need I say it? Ok.. so freakin’ cool.
Another one I’m linking to on my del.icio.us page, but too cool I had to blog too:
ColorZilla Extension for Firefox and Mozilla
Gives you a little color picker in firefox so if you hit a page and you just have to know what that color is on the site, you can use the picker right in firefox to find out.
Usually, I use colorpic for that, but this is handy if you don’t want to start up another program.
I just installed this FireFox extension, and it is pretty cool!!
ImageZoom
Lets you zoom in/out of images on a web page. Granted, an image’s resolution is an image’s resolution, but there have been many times I wish I could get a closer look at an image - even if it did get a little pixelated.