Firefox 57 gained a ton of good will from a lot of users, and they pull this crap right after. They absolutely should know better. They should have known better with Pocket; they should have learned from Pocket.
"Fork it" is not an acceptable answer. The problem is not with Firefox, it's with Mozilla. Mozilla is a good company at heart and they're an important pillar of the web. Losing them to stupid stuff like this sucks, we should fight for them. There's tons of Firefox forks, none of them get the point though, you might as well use Chromium. If Firefox disappears and the fork remains, the fork dies because maintaining a web browser is work that needs a corporation's backing behind it (or a government's).
Mozilla's role goes beyond the web browser as well. Its mission was to "keep the web open", "keep the web free". This goal was reflected in projects such as Firefox OS, Hello and Persona (and to some extent, Thunderbird)... but atrocious management made those projects a waste of time and money.
It's not Firefox you need to fork, it's Mozilla.