This patch was changed on 30d1d481: https://salsa.debian.org/mozilla-team/thunderbird/-/commit/30d1d481403f6fadd363da6796839582fa4750e0#533b8cf491d78f20026b4b74e666a86d89407ab8
On thunderbird 78 toolkit/xre/nsXREDirProvider.cpp had no MOZ_BACKGROUNDTASKS blocks, and it wasn't an issue. (note that looking just at the patch is slightly misleading, since it shows a #ifdef and a #endif which are not paired) Both patches apply to nsXREDirProvider::GetFilesInternal() but after 30d1d481 the LoadDirIntoArray(mXULAppDir, kAppendSysPrefDir...) appears inside the MOZ_BACKGROUNDTASKS. The patch by Михаил seems the right way to solve it, by moving that line just above #ifdef MOZ_BACKGROUNDTASKS. The patch itself is not consistent since, even if MOZ_BACKGROUNDTASKS was defined, kAppendSysPrefDir is only used if MOZ_BACKGROUNDTASKS but declared always (I expect this would cause a compiler warning). Cheers
