I'm in the same situation, and I was advised (by FacialTeam) to wait until my braces come off before getting FFS.

The reasoning was, the braces are likely to change the profile of my face, which then affects the amount of correction that needs to be done in surgery. If you're just doing the upper part of your face then it won't matter, but if you're also looking at any lip/chin/jaw work, it's best to get the braces first and then get FFS once the braces come off.

I asked DBs office this exact question and they said that they do perform surgery if you have braces. But this may be different depending on the surgeon.

I had FFS with my braces on.