Rated 5 out of 5 stars

Great AddOn - I was looking for such a timer/clock for quite a while now. One small thing I'm missing is 24hour mode when specifying the event-time at a notification - but that´s only a bagatelle - thanks so much - clear 5/5

This review is for a previous version of the add-on (1.11). 

Mozilla bug...

Unfortunately this is a Mozilla bug which was introduced with Firefox 4, and I reported: https://bugzilla.mozilla.org/show_bug.cgi?id=637234
It has been fixed, but wont appear until Firefox 7 (tentative release date, end of September. Firefox 7 is currently in the Aurora channel).
At that point, the time selection elements used by Simple Timer will use the time format (12 or 24 hour) defined by your computer's operating system.