Currently the best... but it could still be better.
This addon has everything I want from a 'download to epub' addon - when it works. Unfortunately, the jar sometimes fails to start with no error messages, and the 'grab link' and 'grab tabs' options also seem to fail randomly; - zirusmusings.com, wwyxhqc.wordpress.com, and cardboardtranslations.com all fail in 'grab tab' or 'grab link' or even 'paste URL'. They *do* work in 'grab page', but man. If you're trying to download a 700-page story, who has time to open every page in a new tab and manually grab them all?
The addon I was using after Quantum broke this one is much less feature-rich, but it did at least basically let me grab tabs. I couldn't edit the book before saving, though, or set a title or cover art. I might go back to this one, because all that is great, but I'd love to see it be more consistent about grabbing links and tabs. The old version seemed to handle this stuff fine, so... yeah.
TLDR: Great and feature rich - when it works.