25/01/2025: Thanks to the hint by another reviewer, I downloaded the xpi file for 6.0.0.4 from the previous versions at mozilla.org. I visited about:debugging#/runtime/this-firefox and used the "temporary load" feature. Works, but the trick needs to be reapplied everytime I restart Firefox (or the laptop of course). Note that the Chrome extension seems to work OK with no fiddling required, tested with Chromium on Ubuntu. At least in my experience, raindrop is much slower than diigo, that's the reason I accept the current issue with Diigo.

---------------------------
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."