I’m glad you posted this, as the original poster of 160864, the libreoffice bug did start with an upgrade to plasma6, but I didn’t see how that might be related really, though obviously is. I’ve not run into other apps with similar issue, and don’t currently use thunderbird as you indicated.
Testing this morning using the “SAL_USE_VCLPLUGIN=gen libreoffice” does seem to resolve my issue for now (though makes libreoffice an eyesore), at least a reasonable workaround until their fix gets out to packaging for the kf6 problem.