http://bugs.winehq.org/show_bug.cgi?id=14771
--- Comment #18 from André H. nerv@dawncrow.de 2010-09-26 11:12:09 CDT --- i got the problem here: Firefox searches its prefs in hkcr. HKEY_LOCAL_MACHINE\Software\Classes and HKEY_CURRENT_USER\Software\Classes should exist and be merged into HKEY_CLASSES_ROOT since NT, with prior to the user classes(says msdn). see also the function RegOpenUserClassesRoot, it should be tested, implemented and used by the other regfunctions. I'll write tests and start implementing the function when i get time to.
You can test that by manually copying the user classes to hkcr after installing firefox