I have used Gmail manager forever. Unfortunately, the original creator no longer updates it, as far as I can tell, so it has fallen by the wayside. It was the perfect GMail Add On. I have tried every other option for GMail and the all suck. None of them work like Manager did. I imagine that the changes Google kept (keeps) making to mail made it impossible to keep up anymore. It's a shame we have been left with no valid GMail add-on options.
I have 3 Gmail accounts which I have tried to enter into the so-called GMail Manager, but the Add-on can't seem to remember any of them!! The support site is blank! The Mozilla forum has lots of similar reports of this not working. This seems to be an abandoned Add-On!!! I don't understand why Mozilla lists this as TOP RATED!!!
Seems that since I upgraded to FF33, Gmail Manager doesn't recognize my gmail account at all, like if it was gone, but it works fine from gmail.com so its not my account, maybe something got messed up between FF30 and FF33
Should be looking at forked addon https://addons.mozilla.org/en-us/firefox/addon/gmail-manager-ng/ that is a bit more maintained ; fix for FF32 in GitHub issue: https://github.com/nedwidek/firefox-gmail-manager-ng/issues/47#issuecomment-54694514
For those saying it doesn't work, it's because it was designed to show in the addon/status bar and they removed the addon/status bar in Firefox 29/30ish. I installed Status-4-Evar and it shows up, you might also try some other extensions that re-add the addon/status bar. I also have "The Addon Bar (restored)" but I installed Status-4-Evar first so it's showing up there I think.
Although I am having problems with it lately, It's showing the error icons, I'm not sure if something else is logging me out or what. I have to click logout all accounts and then relogin for it to work, but then it shows an error the next time it checks, so it's like it's not staying logged in. It probably doesn't help that I have multiple accounts.
I love that it shows me the subject lines of the most recent messages when I hover over it, I wish other notifiers would do that instead of just the number of unread messages.