According to this page, Thunderbird supports the lockdown of settings:
Therefore the pref extension must be enabled at build time. This is a very
useful for enterprise environments.
Since this bugzilla report was filed, we have seriously upgraded Gecko-related
packages in Rawhide, which may have resolved this issue. Users who have
experienced this problem are encouraged to upgrade their system to the latest
version of their distribution available.
Closing this bug as CANTFIX. Please, reopen, if this bug is still reproducable
on the latest update of your distribution.
[This is mass-closing of bugs which seem to be too old and irrelevant anymore;
we are sorry, if we are closing your bug in mistake; please, don't hesitate to
reopen, if it is still alive issue.]