i want to ask why cannot view a gif file

i created something in photoshop and saved it as a gif file. when i try to preview, it works good just the way how i want it to be. then save it. but the problem is when i try to open the saved file nothing shows in the mozilla browser. i did it many times before. no changes made in the setting. but why suddenly i cannot view a gif file. can anyone please help. thank you.