In Firefox, it's an SQLite database in the profile folder, readily accessible by normal SQLite tools. The profile folder is accessible through the Help menu, if you don't like to dig for it in a file manager.
You can also export them to json from the bookmark manager, I do that semi-frequently so I can "spring clean" my bookmarks.
My old ones are still backed up and can easily be grepped with no external tools if I need them.