I am flying into Frankfurt on the 26th, and then taking a train from the airport to Rothenburg. My plane lands at 7:20 AM. The train options are 8, 8:30, and 10. Would 8:30 give me enough time?
Not likely. Take the 10:00 train. It normally takes at least 15 minutes just to get off the plane, let alone get through passport control, baggage claim, then find your way to the train terminal. You probably don't need to purchase the ticket in advance - so if by chance you get lucky and get in early, you may be able to catch the 8:30 train.
It looks like you want to take the 8:37 from the FRA Fernbahnhof to Würzburg. If your plane lands on time, that should give you 1:17 to get off the plane and to the Fernbahnhof. That should be enough time. I think that in the past, I have been able to get off the plane and out to the Fernbahnhof within an hour of landing. But then, I have only a carryon bag - no need to wait for baggage and customs.
But, your flight could be late. I wouldn't buy a non-refundable ticket for 8:37, or even 10:00 for that matter.
One the other hand, there is an S8 leaving at 9:09 from the closer in Regionalbahnhof. You would have an extra half hour to make that. With a 19 Bayern-Ticket-Single (I assume you are alone) and a €6,75 local ticket, you could take that S-Bahn into Frankfurt Hbf. That S-Bahn comes into the upper level of the station at 9:21 just a few platforms down from a Regional Express that leaves Frankfurt for Würzburg at 9:30. You would get to Würzburg at 11:24, in time to catch ...
the same regional trains to Rothenburg that you would take if you wait for the 10:01 ICE from the Fernbahnhof. That way the fare would only be about €26. The local ticket covers you to Kahl, the first stop in Bavaria; from there the Bayern-Ticket-Single is valid.
Thank you to everyone for your advice. I leave in less than 2 weeks and I can't wait.
One thing that will matter somewhat is at what Termainal you will arrive. Lufthansa and most Star Alliance airlines arrive at Terminal 1, where there is good access to the train stations. However, some US airlines arrive at Terminal 2. I would guess that it takes an additional 15 minutes to get to the trains from Terminal 2.