Yeah, @lah7 and @marquiskurt, I got deja vu from your comments above, respectively:
...and:
I remembered this post from only about a month ago where somebody complained about Brave having flat, gray "Material" folder icons, and somewhere in the conversation this Reddit article came up which gave a workaround:
https://www.reddit.com/r/techsupport/comments/4xgyqz/my_google_chrome_bookmark_folders_have_changed/
So much for that workaround today. It doesn't work now. Of course Mozilla doesn't have the resources (or the intentions) on keeping a "classic" user interface alive across browser generations. If they did have the intentions and resources, we could still use Firefox 89 with a Phoenix user interface 18-and-a-half years later. 
The very classic user interface is one of the selling points of the Pale Moon browser, but it's got its own set of problems which are far, far harder to tolerate than an occasional user interface change in Firefox (at least IMHO -- probably someone like @jymm has a differing opinion).