Pale Moon is derived from Firefox. It looks & behaves like earlier versions for Firefox; circa version 24.
The last several versions of Opera are based on Chromium. It has serious lack of support for managing and importing bookmarks. The last decent and native version of Opera as 12.16 (14 months old), and the bookmark management is quirky in that one as well. If you want to use Opera, go to filehippo.com and download that older version.
Ya.. the Opera BOOKMARKS were a pain.. :/