Mostly works (well enough) Rated 4 out of 5 stars

TabAlarm doesn't seem to run a timer function itself--only checking the time when there's a "trigger" (such as a display change in the browser); which means that if you set a tab alarm and aren't actually using the browser at that time, then it may not trigger the tab/site you want to open (at least, it doesn't for me). This is easy enough to get around, though, by actually running a timer (such as the CountdownClock extension) which can update its countdown display in the status bar on a continuous basis. (Also, the Edit UI doesn't accurately reflect a time being AM versus PM.)

This review is for a previous version of the add-on (1.3).