2) Report those sites to https://webcompat.com/, and/or to Mozilla (who have an evangelism team to reach out to those sites and get them to stop doing that).
I just keep a Chrome install around and when a website seems like it's misbehaving I just fire it up in Chrome.
It's a bunch of baggage to have around, but it's useful for other stuff. Like when you hit your monthly limit of free articles you can just fire it up in Chrome and now you've doubled your monthly free limit!
Sometimes this might not work, in that case try a different extension or different user agent strings. I had to go through a few before I found one that worked.
2) Report those sites to https://webcompat.com/, and/or to Mozilla (who have an evangelism team to reach out to those sites and get them to stop doing that).