It used to have a nice preview of what was being clipped.
I think it’s been gone since the very nice addition of clipping HTML was added.
It’s missing from my install on latest firefox on windows 10 anyway.
Also it would be lovely if the clipper would work with Reader View pages, which does a great job of removing cruft/ads etc already. At the moment it doesn’t work due to “host permissions” which, is somewhat beyond my understanding.
Extensions don't have access to certain special pages like reader view, config, etc. But Clip Simplified option uses the same code as Reader View, so you could just use that on the regular page.
For the preview, it was working on maybe 90% but on the remaining 10% it was causing major issues so I had to remove it.