Hi all. Disclosure: I’m the developer (Quiet Mooring Software).
ArticleFerry is a free add-on that turns the article you’re reading into an EPUB 3 entirely inside Firefox: Readability extraction, a generated cover, title/author metadata, and images embedded where you allow it. There’s no server, no account and no analytics. It can then open your mail addressed to your Send-to-Kindle address so you attach the file yourself (Amazon has no Send to Kindle add-on for Firefox).
I’d value feedback on pages that come out badly, and any questions about permissions or data handling. Thanks!