Good functionality, but ugly button Rated 2 out of 5 stars
Wel first thing to say, it work as it have to. Great. But what I don't like about this add-on is the BIG UGLY toolbar button. With update notifier you had a small one. That was really nicer (in my opninion). Now a day I use it on my crap machine, and on my "normal" machine i update Thunderbird by hand not with this Add-on. When ther will be a small button, I will use it also on my "normal" machine.
Last but not least, Keep up the good work (and try to make the button compact in the future).
Another user (rob64rock) created a userstyle that hides the drop marker and menu until an update is found which makes the button look like the one from Update notifier http://userstyles.org/styles/53823/fx-6-hide-ext-auc-button-dropmarker-if-not-needed . You can use Stylish or userChrome.css to use it. I'm using that specific button style (built-in to Firefox) so you can just click the button instead of having to go to a sub menu to check for updates. I will not be changing that.