janpeeters I didn't do anything except for installing it from the Firefox extension store — this extension specifically: Readwise Highlighter. It's worth noting that it didn't work on tabs loaded before the extension was installed, but new tabs worked as expected.
As of today, the Firefox extension works correctly on:
Version 0.99.131.0.1-beta (WebKit 621.1.2.111.4)
Build date Feb 25 2025