I'm another person that sits on the floor when in person. When I first starting seeing T, she was in a clinic and we often used different offices. Some where better than others. And some of those we'd sit on the floor and others we would not. When she had her first private office, I mostly sat on the floor because we'd move our therapy there. I liked the couch there the few times I sat in it. She moved offices and got new furniture that is all leather or fake leather. I really dislike leather furniture. I've only tried to sit in one of the chairs one time. I hated it.

Bring it up to your T perhaps she has a chair available or can bring in one from the waiting area, bring your own chair if there is space, ...

I guess it depends a bit on which ways the chairs are uncomfortable and if there's a way to make them more comfortable.

If I was to meet my T in her current office for the first time, I wouldn't have even given her a 2nd visit as the arrangement is so to my disliking. If you are new to this T and have not really created a connection - I think it is a fine reason to look for a different T.