Shaykh Wasee Allaah said, "It is not permissible to sit in a chair (for prayer) if one is able to sit on the floor because this (sitting in chairs) was not established by the Prophet (sallallaahu 'alayhe wa sallam), nor did the Companions do this. But if a person has an excuse (ie. he is not able to pray sitting on the floor), then there is no harm in it."
Additionally, if a person must pray in a chair and can not pray on the floor, then he should line up according to his shoulders, not feet. This is due to a very practical reason, that if he was to line up with the others with his feet, then he would be behind the row actually and his chair would block the person behind him from making sajdah, and Allaah knows best.
Moosaa Richardson
******************** سبحانك اللهم وبحمدك أشهد أن لا إله إلا أنت أستغفرك وأتوب إليك