According to this page, Thunderbird supports the lockdown of settings: http://www.int-evry.fr/s2ia/user/procacci/netscape/en/mozilla-autoconfig-en005.html 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.]