It is easy, but not intuitive, to import bookmarks from Safari into Firefox. First, export Safari's bookmarks using the "Export" command from the File menu. Then in Firefox, open "Manage Bookmarks" from the Bookmarks menu. Now you can choose "Import" from the File menu and get the option to import from a file.
It's strange that the option to import from a file is not included in the import window you get from the main file menu. That is where most people would look first.
Is it time to ditch Safari?