Pretty futile Rated 3 out of 5 stars
The addon is good. But lacks many features.
It does not hook into the Firefox Sync API, so needs a separate account.
The development is too languid... FF4 compatibility was added on 4th June, 2011! 3 months after FF4 Final release! and just 20 days before FF5! This guy is too slow.
And it does lack many features like enable/disable remembering, themes, extension settings etc... All in all a lacklustre addon with an admirable goal.
And most of all, his efforts are imminently futile. Theses features are already on the Firefox Sync roadmap: https://wiki.mozilla.org/Services/Sync/FxSync/Roadmap