New 4.x - 7.x version Rated 5 out of 5 stars
In my experience just changing the version string didn't work. A huge block of icons appeared in the toolbar, the tab groups didn't work correctly, etc. I've fixed most of the glaring issues and, lacking time and any firefox theming experience, added stuff from the default 4.x theme to make certain things work.
I've been using this on firefox 4.0 for months (shortly after DonGato stopped maintaining this), with little issue, however I've only just installed firefox 7, and was actually surprised that it seems to work without additional problems.
It's available at http://code.google.com/p/whitehart-updates
If people are still wanting to help keep this afloat (seftali, I cannot find your email), do contact me at the google code page, report problems or post patches via 'issues', etc.