On my MacOS 10.15.7 system I upgraded from 125.0.2 to FF 126.0 this afternoon. When I started the new version of FF and acknowledged the "downloaded app OK" dialog all the tabs opened as expected except for WordsWithFriends (on Facebook). WWF draws the full board just fine but then throws a less than descriptive error message: "an error occurred. Please refresh." That is an infinite loop as it will return the same error no matter how many times retry is pressed. I logged out of FB and back in, just in case. No change. FB pages show fine, btw. Closed 126.0 and tried downgrading back to 125.0.2. That version with no longer load (or any of the previous four I had on hand). Consumes 100% of the CPU for about six minutes and then changes to Unresponsive in the Activity Monitor. WWF works fine in Safari and Chrome for what that is worth.