Do you mean you want the mobile bookmarks to replace the desktop bookmarks? I can't think of a built-in way to do that automatically because Firefox keeps "desktop" bookmarks and "mobile" bookmarks in separate compartments. I think if you are able to Sync your iOS bookmarks to desktop, you should see those in the Mobile Bookmarks category in the left column of the Library window ("Show All Bookmarks"). Then you could select/copy/paste the entire list from Mobile Bookmarks onto the main Bookmarks Menu or Bookmarks Toolbar, which are what show up in your desktop Firefox. And then after that, you could multi-select/delete the old bookmarks you don't want any more. (Or maybe it's easier to delete the unwanted ones first and then copy/paste?) Since this is all theoretical, hopefully someone with more hands-on experience can guide you on the details.