Comment by kevingadd

Mozilla has had their own dedicated license - the Mozilla Public License - for as long as I remember. My understanding is that FF and Thunderbird's source code are both still under this license.

https://www.mozilla.org/en-US/MPL/

Whether or not the MPL counts as 'open source' is a question for the people who steward that term, I guess. But they've not been using a Standard Open Source License for a while.