Wow! Rated 5 out of 5 stars
Hey this is awesome. Works fine so far too.
I was only going to give it 4 stars because I don't see compatibility for Firefox 3.5 (due out any day now), but it does work in 3.0 so that's the right thing to do. Please do update it though.
Apparently just updating the max version number allows TabTimer to work in Firefox 3.5 though you may want to do some comprehensive testing.
Please add the ability to edit added events. I chose the wrong time and had to delete the entry and create a new one instead of just being able to edit the time.
Still love it though!