At least in both versions of Firefox I've used, there is in the Bookmarks pulldown a "Manage Bookmarks" command, which in turn fires up a window whose File pulldown has import/export commands. Export your old bookmarks to an HTML file, copy it over, import it again.

I've done this; what I can't do is persuade the bookmarks manager to rearrange the bookmarks order the way I want. It says you can drag them from place to place, but they wind up moving the wrong way.