There are useful context-items listed when you right-click on any bit of a page. One I've particularly enjoyed, is "View Selection/Page Source". If I like the design of any widget I notice, I can easily view it's source, copy it and embed it into my page. I wouldn't really call it stealing, cos widgets are public, right? Go ahead, hack and rip my widgets' designs if you can!
An extremely unique add-on, that I've used for quite some time, is...
Forget middle-clicking on every blogroll. Select the entire blogroll with one right-click, release, they're open! Power-browsing made easy.
*LOL* @ IE and Safari users! You have to pay for "Navier", the IE alternative.
Go Get Firefox with Google Toolbar for better browsing For Free!
Have you ever come across a term(inology) that you needed to Google? Of course you have! Another cool item is...
"Search for selection using Google"
The Google search box, next to Firefox's address bar, controls the search method used. There are numerous search engines to be installed and used from within that specific search box for firefox.
Now search for torrents from my blog's new search box!