Quite often you come across a page that already has a bookmark, but you want to see it within the bookmark folder hierarchy. At the moment this is quite difficult because you have to search through your bookmarks.
The most intuitive way would be an option to locate the bookmark inside its folder. Similar to what you can do when searching bookmarks and opening the context menu, but in this case available directly from the bookmark icon next to the URL.
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BTW, when using FF mobile on Android, new bookmarks are always stored to the folder last used in the desktop version, without an option to change that apparently. That makes saving bookmarks on mobile quite useless, because it's nearly impossible to locate them later ...