Rated 5 out of 5 stars

Incredible!
Very well thought - almost every detail of it.
The only flaw so far is that it doesn't detect plug-in activity - ergo, it auto-locks and/or auto-logs out after the set amount of time even though you're watching a video / playing a flash game / whatever.
Other than that - it's very useful for example, if you want to protect the information in your browser while you are at work and you can't be on your computer all the time.

All in all - fantastic add-on!

This review is for a previous version of the add-on (1.15.1.1-signed). 

It's a known Firefox issue (or you may call it a bug even).
My understanding is that Flash is a external application that doesn't share user's input events with Firefox.