User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:85.0) Gecko/20100101 Firefox/85.0
Steps to reproduce:
In past versions of Firefox, all new bookmarks were added in the same folder -- "Other Bookmarks". This behavior was recently changed in https://bugzilla.mozilla.org/show_bug.cgi?id=1665389, but the change is larger than the bug description indicates: not only the default was changed, but Firefox now remembers the last bookmark folder used, and uses it again for the next bookmark.
I think this is a bad idea. There is no reason to assume that the new bookmark has much to do with the previous one, and indeed I have put multiple bookmarks into completely wrong folders due to this change in behavior.
I tried setting browser.bookmarks.defaultLocation to "unfiled", which works exactly once, but when I change the bookmark folder on one bookmark it still also changes the default for other, new bookmarks. So currently there seems to be no way to have all new bookmarks appear in the same, consistent folder, which IMO is a regression compared to earlier versions of Firefox.