During the month of Ramadhaan a musalli would come early to the Musjid and place a row of chairs in the saff.

Answer

Assuming that all the chair-sitters are invalids and are genuinely unable to perform Ruku and Sajdah, then too it is not permissible to give the Musjid the appearance of a church or haraam marriage hall. The chairs should be scattered and generally placed at the extremities of the rows. The manner in which this musalli arranges the chairs in the saff is not permissible. He should be prevented from this obnoxious act.

If a person is able to perform Ruku' and Sajdah normally, then it is haraam to sit on a chair for Salaat. The Salaat will not be valid. If he is able to sit on the floor, then too it will not be permissible to perform Salaat on a chair. His Salaat will not be valid.