This is the first I've heard about Tab Groups being removed and that makes me sad. However the comments in the bug tracker [0] make it sound like it isn't about the money. They also have a migration plan for existing users to back up their data and move on to an extension [1].

[0] https://bugzilla.mozilla.org/show_bug.cgi?id=1222550

[1] https://github.com/Quicksaver/Tab-Groups

[1] https://addons.mozilla.org/en-US/firefox/addon/tab-groups/