Updated: works on FF22-24+!! Does not work with "Flashblock" installed. Rated 5 out of 5 stars

It works!! Thank you very much, Mikhail, and I apologise for not properly checking my browser before saying that it didn't work. The culprit is Flashblock: when I enable that add-on, Regain Focus stops working for me. Otherwise it works perfectly! Firefox 22 with Noscript, Windows XP. I had set Noscript, Flashblock, Greasemonkey, and a few other add-ons to accept all Javascript, Flash, etc.—that is, I had done everything except disabling them in the add-on manager. I did that to test why it didn't work; and it didn't work under those conditions. But it turns out it was not enough to set Flashblock to "accept all scripts": Flashblock actually has to be disabled in the add-on manager for Regain Focus to work, at least on my Firefox.
Thankfully, we don't need Flashblock any more: with Firefox 24, you can pick "ask to activate", en then, where there is Flash on a page, click it and pick "always allow Flash on this website", at last!

This review is for a previous version of the add-on (0.2.1-signed). 

addon works fine

I just updated firefox to 23.0. This addon still works fine.
Which operating system you are using?
Did you try to press Ctrl+T when focus is in flash? (It works.)
Which site you are trying?