'Tired' is a single syllable adjective, so 'more tired' is correct. You may hear people say 'tireder' informally or colloquially though as we don't always follow the rules!
As thirsty contains two syllables, 'thirstier' is more appropriate. Again, people may say 'more thirsty', but it's not strictly correct, because...English!
Hope this helps, thank you for your question :)