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.