There are those. I pay to support Floorp.
But I also donate to Firefox and Thunderbird cause the forks wouldn’t exist without them.
But I also donate to Firefox and Thunderbird cause the forks wouldn’t exist without them.
Your second paragraph is more important though: none of these let's call them Firefox distributions are proper forks. They're not developed by teams who could develop and maintain a browser independent of Mozilla. I believe that's what your parent comment means by a "serious fork". Floorp and Waterfox and LibreWolf are not that.
They do good work. I can forgive them for not being the best at naming things in English.
To Thunderbird, however, we can actually donate to.
Though, just because money goes to the corp, doesn't mean it will contribute to Firefox' development either.