Obviously i love greasemonkey, one of the best ever extensions of firefox. Here is perhaps the best ever.
firebug from mozilla or from firebug website

What does it do?

1. Install firebug just as you install all other extensions.
2. Enable firebug. (press F12 and click enable firebug), Alternatively, you may also use Tools-> Firebug -> Enable firebug
3. Now visit your website or blog.
4. Press F12 or click the small green circle in the right corner of the status bar.
5. This section must show you, the individual time, each component (made by a separate request), took to load. This can be of great help to tweak your page to load faster. If a small section of php code added in your blog as a script takes so much time, remove it.

Obviously, this is just a new and additional feature in firebug.

To use it to the full power, you may like to click on inspect and move your mouse around the page. Magic??

It may often occur to you that you want to edit your css and look at the results. Dont make the changes on the template, rather you can make it in the firebug screen to see dynamic changes. Find a nice template in your friend’s blog? just press F12 and inspect the code.

This tool comes in extremely handy when writing greasemonkey scripts. If i were to vote for the best mozilla extension, my vote goes to firebug!!