Rated 3 out of 5 stars

1. I don't like the system with check intervals. In my opinion - every tab should have some internal timer, which starts counting time since the tab became inactive. And when that timer reaches extensions.dormancy.TabDormancyAgeMs value - the tab should be put to sleep
2. I use Tree Style Tab and your extension breaks trees, when putting tabs to sleep. Sometimes, they get back to normal trees, when re-activating the tabs but only sometimes...
3. Titles of the hibernated tabs look like this: data:text/html,<title>The actual tab's title</title>, but when they get restored - usually their titles get fixed, but sometimes they do not.
4. Hibernated tabs have crop=center instead of crop=end.
5. Bug 675541 should be marked as dependent to the Bug 675539. This bug is about styling the stalled tabs, I suggest adding opacity: 0.8; to them.
6. Sometimes it wrongly stalls a pretty fresh tab (e.g. I'm sure that the tab was too fresh to be stalled: it was inactive for like 3-5 sec [I just switched from it to another tab] and the Age pref is set to 300000.

This review is for a previous version of the add-on (1.0.1-signed.1-signed).