MB 75: Use Mullvad name (fix for mozconfig-linux-x86_64-dev)

Pier Angelo Vendramerequested to merge
pierov/mullvad-browser:75-mb-fix-mozconfig into privacy-browser-102.8.0esr-12.0-1

I had lost one final pb- in the previous MR, and tested only in mullvad-browser-build tor-browser-build-s131, so I didn't catch it either.

Related: #75 (closed)

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