Pwn2Own came and went and Firefox fell with it. The __proto__ vulnerability seems exploitable in TenFourFox, though it would require a PowerPC-specific attack to be fully weaponized, and I'm currently evaluating the other bug. Builds ("FPR13 SPR1") including fixes for either or both depending on my conclusions will be issued within the next couple days.

http://tenfourfox.blogspot.com/2019/03/stand-by-for-urgent-security-update.html