I hope they got their act together on profiles. Was not amused with their last upgrade of having to backup all my saved mail and create a whole new profile.
Why? Was starting with --allow-downgrade not enough? Because for me it was.
Already did that while that file still was set to build 78.4.0, only had to disable the with-system-bz2 option in the PKGBUILD to get the build working. Works like a charm.
Just updated the PKGBUILD from svn, chose to overwrite my changes, updated the PKGBUILD to 78.4.2 and build went through without issues.
Thunderbird still working without issues, too.