Enable 'video background play fix' extension.
View a long video for example https://m.youtube.com/watch?v=5gnlhmaM-dM
Lock the screen and continue listening.
After some time unlock the screen. 5 or 10 min should be fine.
I can see the web page

everything is white

This is a regression. Works fine in beta.
I have the same problem... It's annoying!
Here is a logcat if it helps https://my.owndrive.com/index.php/s/w2NcqJJpRfkMB7G
cc @csadilek (or is it enough to put the WebExt label?)
Talked to @agi and moving this to Bugzilla.
Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1672782
Change performed by the Move to Bugzilla add-on.
this stopped reproducing