How bout you learn how to use DDU to make 4k downsampling work.

First of all, whenever I personally encounter crashes in Firefox, it's because of an extension or an enabled setting that really shouldn't have been. Second, seeing as I never heard of Waterfox, I'm guessing it's not exactly based on stable releases. So, I wouldn't be too quick to point fingers at the drivers unless you have proof that older ones worked. That doesn't mean alternative drivers won't crash, but, that doesn't mean it's AMD's fault either. You can't judge a car's safety by driving off a cliff, so you can't judge the stability of drivers when using unstable software. I don't know what MPC is so no comment there. If you find yourself with several programs failing to run, you may want to wipe clean all of your drivers and do a fresh new install. Same goes for Firefox itself; there have been a few times where Firefox was giving me issues and the only way to fix it was to reinstall it. Weirdly enough, keeping my extensions didn't repeat the problem.

I can tell that I use Waterfox for more than two years now, it has never failed on me and it is as much stable as the original Firefox if not even more stable. I have never had any problems with Waterfox, I use it everyday as my default browser and I recommend it for you to try it too. Hope that I have helped a bit clearing the confusion about Waterfox.

For me its actually much slower than previous releases.I tested with far cry 4 ,3d mark11 and kombustor they all runed slower.

This is weird. Have you done a clean installation?