Thanks for that I thought I was going mad as it stopped working, then the next day it worked, then it stopped. All the videos I have tried to bookmark recently - nothing happens when I click on CLIPPER (in Firefox, Edge or Opera on several PCs) you cant't even selct the bookmark or screenshot instead of YouTube, in CLIPPER, as nothing pops up any more.
This explains WHY some videos worked and others not.
I have foun a workaround by using TOGGLES https://toogl.es/ or VIEWPURE http://viewpure.com/. Watch the same videos there and you can at least bookmark them.