Regarding the errors when exporting to Safari, it is true that this can be caused by subfolders having the same name. I just discovered this yesterday myself. This applies to both Reading List and the root level,
as explained here.
Regarding the mapping of Safari
Reading List to Firefox
Other Bookmarks, I agree this does not make sense today. It made sense when we originally designed this. Until a couple years ago, Firefox
Other Bookmarks was called
Unsorted Bookmarks. They changed it, presumably to play with the
Other Bookmarks of Google Chrome, but they still have a
Bookmarks Menu, which Chrome does not. And we've always mapped Chrome's
Other Bookmarks to the
Bookmarks Menu.
So, although it makes sense to old users, this has indeed become a mess for new users, and we should have a way for users to change the mapping. However, we're not undertaking any major initiatives until after next week, when Apple is scheduled to make their annual announcement of how they plan to blow things up in macOS and Safari.
What I recommend for now is to make and use your own folders, and put them all in the same hard folder.
For this hard folder, I recommend the
Favorites (Safari),
Bookmarks Bar (Chrome), and
Bookmarks Toolbar (Firefox). You can hide this toolbar in all browsers if you don't want it taking up space at the top of all windows.
As a second alternative, use the root level (Safari),
Other Bookmarks (Chrome) and
Other Bookmarks (Firefox). This has the advantage of easier access in iOS devices (because iOS shows the root level when you tap the Bookmarks button). In this alternative, you are subject to not having duplicate-named folders as explained in that other post I linked to. But duplicate folder names is kind of not a good idea in any case.