This has been been going on for a long time, with each update another site no longer works properly with Firefox. So I switch to Chrome and carry on, there being little point in complaining. Of late it seems that so many sites no longer work that at the point of giving up on Firefox. I haven't kept a log of problem sites as I I'm usually trying to get something done and I don't have the time to dig into it. Try this one https://www.buydisplay.com/ and see if it works for you. For me the menus open then close before I can click on them. Works fine in Chrome. I'm on Win 10 64 bit Firefox90.0.2. Only plugin is Ghostery. Disabling that doesn't help.