You can't access about:config in the release version of mobile Firefox. Only in the beta or nightly builds. The beta version is reasonably stable to be fair although it can fluctuate.
I discovered mull only about a month ago, but yeah, its great. As close to Librewolf you'll get on mobile. A limited number of extensions work, but my favs are there. Ublock, NoScript, Decentraleyes, Dark Reader, Clear URLs. It also has access to about:config and is available on fdroid. Great mobile fork.
I just share the download link to my file manager (MiXplorer) and I can pick and choose where the download goes.
It's not exactly what you want and some downloads (with redirects) might not work, but ever since i've used Mix, I use it exclusively to download everything from Firefox.
I only just switched back to Firefox after many years, and have one open with my bookmarks and tabs all the time now! Although I do use Simple Tab Groups for the latter