A very welcome add-on Rated 5 out of 5 stars
Gmail S/MIME makes sending and receiving encrypted Gmail messages a breeze. The installation takes care of itself, and using the functionality is pretty easy to learn -- as soon as you figure out what you're being asked for in one dialog box that wants your "master password" (I assume this means the password for Firefox Password Manager, which I don't use, so I just enter blank here and it works).
The most difficult thing to understand (for me) was obtaining and installing the necessary certificate, but there are various resources telling you how to do that, such as the marknoble.com page referred to in a previous review, or the CAcert wiki pages.
When I did have a problem -- which turned out to be Mozilla's bug, not the add-on's -- the add-on developers responded quickly, informatively, and professionally; told me how to work around the problem; and gave me instructions for how to perform some further tests so they could make sure there wasn't a bug in their code that was associated with my problem. (There wasn't.)
As everybody says, signature verification would be very nice, but until the day when I'm using Gmail messages to wire millions of dollars around the world, I suppose I don't absolutely need it -- and I do appreciate the developers' continued work in that direction. (The sig verification, that is; I don't think they're working on making me an international financier, though that would be a nice add-on as well....)