If it fails with a new user then that implies something missing from the install, IMO.
If the new user works then it is a user setting apparently.
Yup! And there we have it... New user worked just swell.
Since I already tried just the .mozilla folder, it had to be somewhere else, so only one thing for it: go through all the hidden configs on my user.
And you know what? Found it!! Turns out that I had an old .asoundrc file lying on this user:
... from back when I was getting my digital surround sound on.
Without this file, FF plays nice. Now, why this would affect only FF and no other browser is puzzling, but alas... there we have it.
I feel I must offer a slight apology for wasting everyone's time with this - sorry! - but hope that you shall forgive me... after all, the file was
So, thank you all for you help, and especially just18 for insisting - without a new user, I would probably not have found the blasted file.