I believe even if you choose “Open in Firefox”, it will still download the PDF to the default download directory before opening it inside a Firefox tab. The behavior that OP describes above seems to prevent that downloading (and having the file around in your default download directory).
Good question! I suppose the two extensions have very different main ideas/approaches, so maybe merging them isn’t so ideal or feasible. And to be honest, I also enjoy the freedom of having my own project!
Yes, I’m a big fan and very happy with Omnivore as a Pocket replacement. I also built a little browser extension to mimic the practical popup of “In My Pocket”, but based on Omnivore, since I was missing that particular functionality. https://addons.mozilla.org/firefox/addon/omnivore-list-popup/
I don’t think it’s possible. And please change the title of your post to something more descriptive. I just see it as “@mozilla”.