But if the user wanted to keep using native bookmarks, couldn’t they periodically export the bookmarks from both Chrome and Firefox, upload both files to Google bookmarks (which won’t import any it already has).

Now you have an integrated list of bookmarks from both browsers.

Export the file and import* to both Firefox and Chrome.

Now they both have the same bookmarks!

*Maybe erasing all bookmarks in the browser first because I don’t know how the browsers respond to duplicates. So better to start empty. But I’ll test it and see.