I assume that you are referring to Firefox Sync ...

Look for a new folder in your bookmarks on the desktop that is labeled something like "Mobile Bookmarks".

Yes, Firefox sync. I have a mobile bookmarks folder but that has the bookmarks from my tablet, not my iPhone.

There should be a folder of bookmarks from the iPhone in the Sync data that gets sent to desktops / laptops.

tablet -- is that an iPad that uses iOS as the iPhone does? And uses the same "mobile" version of Firefox.

Are you sure that the iPhone bookmarks didn't get put in with the "tablet" bookmarks? I have read that "mobile" bookmarks (phones and tablets) are handled a bit different by Sync than the desktop / laptops versions are handled."Everything" should Sync from "mobile" to "desktop", but not to "mobile" from "desktop"; there is a much smaller limit to how many bookmarks a "mobile" device can have saved; at least as far as how Firefox works on "mobile".

If that isn't the issue, I guess the iPhone Sync was not complete. Try disconnecting that iPhone from the Sync server and try reconnecting the following day, to see if a complete Sync is then performed.

The mobile bookmarks folder has bookmarks from my Android tablet that I don't really use anymore. I have an iPhone and an iPad mini but those bookmarks aren't showing up.

I've tried moving all of the mobile bookmarks to another folder using Firefox on Windows. When I did that the mobile bookmarks folder disappeared. I then signed out and signed back in to iOS Firefox but to no avail. Still no iPhone bookmarks and now no mobile bookmarks folder either.