"You should use an up to date major browser. Old Firefox forks are not supported and expected to have problems."
It's all incredibly telling, that they've given up trying to be impartial. When "they" start picking browser winners and losers, are OS's next?
In a way Cloudflare missed an opportunity, because a try()/catch() around the bit of failing JavaScript would have been perfect fingerprinting. Having said that, I don't expect it will take the Pale Moon team very long to patch the problem.
But where to go from here? Is there anybody besides the ACLU and EFF with enough resources to mount a "public nuisance" lawsuit? And what would constitute winning? A court-appointed overseer to make sure Cloudflare is regularly educating its staff on the variety of browsers in use today, and providing near 24–hour turnaround times when issues like this occur? It would be a start.
Personally I wonder if this whole style of security is a fool's errand and any blocking should be server-based and look at behavior, not at arbitrary support of this or that feature. I think it would also be helpful if anybody who finds themselves blocked would be given at least a sliver of why they were blocked, so they could try rectifying the problem with their ISP (bad IP), some blocklist, etc.