What site? What OS? If windows, home or pro and what security software are you running besides MS Defender?

Here is an example site (but any site presenting maps has either this message or a blank screen) https://experience.arcgis.com/experience/028de5f59b014757bda5cc2444d1f0c9

I have windows 10 pro, 64 bit, version 22h2 up to date.

Worked fine one day and then it didn't. Only difference I can see in about:config are two new lines marked web2gl that are checked as true

The exact same error happens for me at https://www.provo.gov/1014/Live-Map-of-Open-Requests

And I have the same setting(s) in about:config

https://get.webgl.org/webgl2/ says Firefox is ok.

The problem lies with the graphics drivers.

Unfortunately my laptop is over 10 years old. I found https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration#w_using-the-latest-generic-graphics-drivers which had a link to a tool from Intel to check if there were any updates for mine and unfortunately there are not. Windows Updates doesn't have any either.

Oh well, just another reason to get a newer computer :)