And while I'm on the topic of offline pages, let me add another rant: As far as I know all Android browsers, including Firefox, keep commiting the cardinal sin of computer applications. They lose user data.
I cannot count how often I returned to the browser only to find a previously loaded page reloading. Good luck with that if you're temporarily in a dead spot.
Of course most of the time you aren't, but the browsers still lose your data, namely the scroll position. This makes these programs that are, to a first approximation, document display tools eminently useless for displaying documents longer than 140 characters.
Even when Firefox had an offline reader mode - built to read longer documents and with complete control over js and rendering - the scroll position was never persisted. I resorted to copy pasting a sentence to a notepad and using Find in page to resume reading. Ridiculous.
And that's not even mentioning form content and other more application-y state.
Of course I know it's an Android thing; apps don't get to hold on to stuff. So store everything to disk. I'll reload it when I'm good and ready. My podcast app doesn't restart the episode from scratch if I leave it stopped in the background for 5 minutes. I'm sure there are a thousand edge cases, but there's very little evidence of fixing even the simple path.
Imagine a desktop browser pulling shit like that. Again, not just a Firefox issue, from what I can tell none of the vendors think this is important.