Something strange is going on. Check this output:
```
$ /usr/sbin/apparmor_parser -Q -p /etc/apparmor.d/usr.bin.thunderbird | fgrep
user_share
@{user_share_dirs} = @{HOME}/.local/{,share/@{flatpak_exports_root}}/share
owner @{user_share_dirs}/applications/{**,} r,
owner @{user_share_dirs}/icons/{**,} r,
{**,} r,
owner @{user_share_dirs}/applications/{**,} r,
owner @{user_share_dirs}/icons/{**,} r,
owner @{user_share_dirs}/mime/{**,} r,
owner @{user_share_dirs}/applications/{**,} r,
owner @{user_share_dirs}/icons/{**,} r,
owner @{user_share_dirs}/mime/{**,} r,
owner @{user_share_dirs}/applications/{**,} r,
owner @{user_share_dirs}/icons/{**,} r,
owner @{user_share_dirs}/mime/{**,} r,
owner @{user_share_dirs}/applications/{**,} r,
owner @{user_share_dirs}/icons/{**,} r,
owner @{user_share_dirs}/mime/{**,} r,
```
So, Thunderbird should have access to mime-stuff in home via (indirectly) included
`abstractions/freedesktop.org`.
Could it be that's something is wrong with "@{user_share_dirs} =
@{HOME}/.local/{,share/@{flatpak_exports_root}}/share" rule?
