I assumed this was a form of a support forum,
This is not Stardock's official Support process [or even the specific 'support' section of these Forums]. Instead it is a User discussion Forum where some assistance may be available from fellow users who may or may not experience the same issues.
Firefox has long been a problematic program to skin...or to support skinning as, with every version release they manage to reinvent themselves to the point things won't work as they did.
Personally? I gave up even thinking of using Firefox about a decade and a half ago for the simple reason they [Firefox] don't seem to be able to find their bum with both hands.
This will simply be another issue of their inanity, and I expect that user file you mention WAS used happily by Windowblinds for user settings re skinning but is only now broken.
I'll see that Neil sees this thread and can deal with FF yet again.