Changelog available here.
They're not considering it a regression and in theory it should be something that's fixed in Thunderbird 78.4 compilation. Given the proximity of sync at the moment, not going to be pushing Thunderbird 78.4, so that's something that can be tested post-sync if you happen to still have the rust eopkgs.
- Intentional breakage: https://github.com/rust-lang/rust/issues/76480 and related comment
- https://bugzilla.mozilla.org/show_bug.cgi?id=1670579 (reported in last comment that it should work for Thunderbird and latest Firefox ESR)