Just Importing/Exporting them. I know when FF runs for the first time it asks if you want to import data directly from Safari (without having to export first from Safari), I'm sure that wizard can be run again to easily transfer them. Of course, that's not a nice automagical method, but it'll get the job done. Maybe AppleScript could get invovled, but I wouldn't know much about that.
Otherwise, I personally try to keep my Safari and Firefox Bookmarks seperate, for business and personal stuff. However with FireFox being my personal favorite and Safari being used on my iPhone it's gotten tough lately.