Rated 5 out of 5 stars
This is an amazing application, but for some people.. It just won't login to their account!
I found that there is a site to another download, then an updated download, but both of them both send me to a 404 error.
Luckily for everyone here, I found the latest update (from what I know) here!
https://github.com/downloads/ath0mas/mozilla-firefox-addons/gmail_manager-0.6.4.1.6-fx+sm.xpi (It's a direct download, so you'll download it when you click the link.)
It works for me, atleast. And I was having the same problems.
I just wish it was more stylish.