In fact, even if Mozilla did pay a non-royalty-free patent license to include the codecs in Firefox, that license would probably be incompatible with the GPL/LG

In fact, even if Mozilla did pay a non-royalty-free patent license to include the codecs in Firefox, that license would probably be incompatible with the GPL/LGPL, meaning it would no longer be possible to distribute Firefox under its current MPL/GPL/LGPL tri-license. (I don't know the MPL well enough to know whether Firefox would still be distributable under the MPL alone.)