Rated 4 out of 5 stars

Excellent !
Will be a lot better with multiple alarms / SMS reminders support though.

Please everybody, don't forget to vote for these enhancements in Bugzilla !
There are a lot of people complaining here, but only 3 and 5 votes in Bugzilla ! That is nonsense !

Use default alarms defined in Google Calendar, or add support for SMS alarms :
https://bugzilla.mozilla.org/show_bug.cgi?id=389088

Only one alarm can be defined per event (multiple alarms wanted) :
https://bugzilla.mozilla.org/show_bug.cgi?id=353492

This review is for a previous version of the add-on (0.4).