!) Make your profile in an area not protected by XP (Documents and Settings are protected). I recommend a separate physical HD, but separate partition or even a separate folder in the XP partition is fine.
2) Use a nice folder tree like Mozilla likes - E.g. I have my Fx profiles in D:\Mozilla\Firefox\Profiles\
2) Take ownership of the folder (regardless of where it lies - and take ownership of the entire drive if another drive or partition, and at the very least, the entire folder tree in Windows 7 (e.g. if your tree looks like the one I mentioned in 2), then take ownership of the Mozilla folder and all subfolders). Then, assign
permissions fr the group
everyone with full control access for that same tree and all its subfolders.
3) Now you should be able to install Fx 3.5.2.
Uncheck run after installing. Go to
Start --> Run and type
to start it with the profilemanger switch, so that it does not set up an automatic profile. Now you can manually create the profile, pointing to the existing folder. Note - you'll need to name it the same as it is already named in your other OS.
A couple of things to consider: If you only want to synchronize bookmarks and settings, look at weave - that way each (OS) install can use its own profile and be happy.
Also, make sure you create more than just a single profile - especially when sharing it across OSs. if you ever get that profile hosed, you'll be SOL without another one in place.
I run Fx final release (currently 3.5.2, plus Shiretoko 3.5.3pre release, plus Minefield 3.6a1 - each has its own profile, and in addition I have 2 more profiles -test ( for testing addons) and plain (no extensions, no customizations) so I can always have a working profile.