FEBE allows you to quickly and easily backup your Firefox extensions. In fact, it goes beyond just backing up -- It will actually rebuild your extensions individually into installable .xpi files.
Previous releases of firefox (well, 1.5 and 2.x at least). Useful for testing against older versions. I've got 1.5 and 2.0 installed and used Profile Manager to create a profile for each. There may be a better was of doing this...
Dead pages, broken links, the scourge of the internet. Powerhouse sites like Slashdot and Digg can bring a server to its knees. What do we do when a page is dead but we still want to see it? (...) use this extension.