Netflix, Youtube not going fullscreen

For about a week now, I have been having issues using netflix and youtube with firefox. I have tried answers provided in other questions, such as emptying the cache and clearing cookies - which works! But only until I leave the page or change what I am watching. Netflix is especially frustrating as it requires me to go to netflix.com/clearcookies every time I use it - which means I also have to re-log in. I have no idea why this is happening and am getting increasingly frustrated. The problem appears in safe-mode (troubleshooting mode) and I cannot find a permanent solution. Any help would be appreciated!