I'm updating this on 12th of June 2024. It is broken on Firefox since version 6.0.0.5resigned1, dated 25/04/2024. The backend still works though. The Chrome extension still works too. 6.0.0.4 was dated 2017. My guess/hope is that it's just a matter of signing again the extenstion, for the author. Crossing fingers... because until it broke, it was one of my favourite tools, and I was a paying customer for the premium tier for many years.

For reference, around 2019, I wrote "The ability to highlight text as I browse Web pages is the cornerstone of my productivity and sanity. I highlight bits of texts and, so doing, I build summaries of interesting stuff, with bookmarks to the pages. So simple and super effective."