Rated 4 out of 5 stars
as todd does not maintain this plugin any longer, you can download latest version here: https://github.com/tomondev/gmail-manager (0.6.4.1.5 is compatible with Firefox 11.x)
compatibility with FF 8.x Rated 4 out of 5 stars
As Todd was not available, I published a fix for FF 7.x and FF 8.x here: https://github.com/tomondev/gmail-manager. You can build it from the sources or download the latest version there from the target folder (gmail-manager-0.6.4.1.3-tomondev.xpi for FF 8.x).
This cool add-on works like a charm under latest FF if you download the fixed version. I can't live without it to manage multiple gmail accounts. Thanks to the original author.
FF7.x: alternative plugin source code and packaged xpi Rated 5 out of 5 stars
Maybe the source code and an issue tracker on github can help us to maintain this great FF plugin! Found this alternative repository (not from the original author, as this plugin was not updated for FF 7.x officially): https://github.com/tomondev/gmail-manager