Web pages with Flash now hang Firefox

[Fulko Hew]

A few days ago any time I try to view a web page that contains some Flash,
Firefox (56.0) hangs for about a minute, then does something, and hangs
again for a minute, etc.  Until I kill the tab for that web page.

Chrome works fine.  Firefox used to work too, but now it doesn't.

Has anyone seen this?

PS. I had originally followed the instructions here to install Flash:

https://www.if-not-true-then-false.com/2010/install-adobe-flash-player-10...

TIA
Fulko