Rated 5 out of 5 stars

by Parkylondon on Nov. 29, 2011 ·

https://github.com/tomondev/gmail-manager/blob/master/target/gmail-manager-0.6.4.1.3-tomondev.xpi?raw=true

"Download it. Open Firefox Menu->Add-Ons
There is a button with a gear icon in the top-right of the tab, click it and select "Install add-on from file" and select the xpi file you just downloaded.
Enjoy."

Thanks to Kutt Katrea on the Mozilla forum page for GMM..

This user has 6 previous reviews of this add-on.