Is there something different about how Firefox handles extensions/addons in Linux? I'm running the latest version of Ubuntu, and every time I try to add an extension, I click on the link, and it doesn't download. It just sits there. This happens no matter what extension or theme I try to install.

Any idea what's going on?

The Ubuntu folks installed a customized (non-stock) version of Firefox on your system, so that they can control the software quality etc..

One result of that, is that many the browser extensions can/must now be installed using synaptic, instead of the usual ways. So fire up synaptic and see if you can find the extension you want in there somewhere.

This is one of the "features" of Ubuntu that I dislike, so I always override their copy of Firefox with the *real* one from mozilla.org, though this is surprisingly difficult to accomplish/maintain for the non-expert.

EDIT: or perhaps you simply need to go into the Firefox preferences, and enable "software installation" (or whatever it's called ??