I’m not sure that’s a great idea. I understand wanting to avoid the hassle of reinstalling Mull and having to go through all the Settings again (I just did that a couple of days ago), so if you want to keep your current F-Droid Mull install I’d recommend installing another updated flavour of Firefox, like Fennec or Iceraven, using FFUpdater (https://f-droid.org/packages/de.marmaro.krt.ffupdater/) and using that one until the updated Mull is pushed to the F-Droid official repository.