Hi k2dfc, are these themes available from the Mozilla Add-ons site? If so, could you mention their names or provide a link to them? Hopefully then someone else can check them out for comparison.

Waterfall is one. The other is one with Snoopy sleeping on his dog house.

Oh, yes, the are on the Mozilla theme list.

Okay, I tried two animated themes and they don't animate for me either.

I found something: Looks like a known problem with Firefox 68, side effect of a fix to prevent flickering when switching windows.

Source: https://www.reddit.com/r/firefox/comments/cbwkzv/animated_themes_arent_animated_since_v68/

Seems you'll have to live with non-animating themes for the next 5-6 weeks.

This happened in the past. Then one day all the Themes got messed up with a new update. A few days later all was fixed. Now messed up again. Oh well.