You're right. I checked with the new Firefox (the one with the "too many taps" GUI from one year ago) and it doesn't work anymore.

I'm using the old Firefox on my main phone and it explicitly says "Saved offline" when I bookmark a Reader Mode page. The new Firefox on the backup phone says "Bookmark saved".

One more reason to stay on the old Firefox.

This was one of the features I was actually sad to see go when I updated Firefox.

I used this feature a lot as it was mainly manuals that I was saving.

They also replaced the default start page to be Collections instead of bookmarks, which b of course for me was empty.

I think I've got used to the new layout that I probably won't downgrade just for the offline capabilities.

Yup, it didn't make the transition to Fenix. That was such a cool feature. Maybe I should downgrade.