Does Xumm support both r- and X-addresses?

XRP accounts can start with an "r" or with an "X". Xumm supports both address formats.

Xumm will display your own account in the "r..." address format. When sending XRP, Xumm supports both the "X..." notation and the "r..."  notation. If people send XRP to your account in Xumm, they can use both the "X..." and the "r..." version, as Xumm supports both.

The "X..." address format is backwards compatible with an "r..." address.

Please note that when sending to an exchange or shared (custodial) wallet and using an "r..." address, you will probably have to add a Destination Tag when sending your transaction.