Rated 5 out of 5 stars

works great, using it with gmail, imo and facebook. proved to be a bit annoying when opening multiple tabs of facebook, but this regular expression fixes it:
^(\([0-9]+\) Facebook)|([a-zA-Z0-9\-\.]+ messaged you)

This review is for a previous version of the add-on (2.0.2.1-signed). 

Thanks for the contribution, pretty useful to avoid the "blinking" of the message, but I tried it and it still shows a notification per tab. I guess the next improvement should be to add a per-domain option to avoid more than one title-notification per domain, or something like that. However, I would insert this and similar suggested regular expressions in the default configuration of the next version, if it wasn't for the problem of different localizations of the websites. So we may just collect tips like this in a space of the support pages.