Flash + Firefox + Ubuntu 18.04 + 64bit = :(

I have Ubuntu 18.04 installed, which is a 64bit operating system. I also use Firefox. On a clean installation I installed the Flash Player and some websites are okay with that - including Adobe's own version checker.

There are a number of websites which fail to load, though. Some say there is an invalid wmode setting, others say RTMP is an invalid URL, others simply show nothing. The same websites work in Windows Vista 32bit Firefox (an old machine to test the same websites) so it appears to be a problem with the combination in the title of this post.

I have checked installation instructions and despite doing exactly as told, nothing seems to make it work, including disabling hardware acceleration and all sorts.

Reading the guide about posting problems, here's some stats:

• Operating system version: Ubuntu 18.04 (fully up to date as of 12 Nov 2018)
• Web browser and version: Firefox Quantum 63.0 64bit
• Flash Player version: 31.0.0.122 (According to Flash Player Help)
• Explain your problem in step-by-step detail if possible: see above
• A direct link to a web page that demonstrates the problem: lots, but here's one: https://apps.facebook.com/angrybirds/?fb_source=bookmark&ref=bookmarks&count=0&fb_bmpos=_0

Any help would be appreciated. I know the Ubuntu and Firefox forums complain this is a Flash issue which can only be fixed by Adobe. This is actually one of those situations where every company is blaming each other.