Is it best to use the YouTube app for Android rather than a browser (some browsers will force redirect to the app. After all, there's a YouTube app for a reason.

There are add ons on Firefox I want to use on YouTube that if I use the YouTube app I cannot use

Did you try clearing the app data from the settings? Install the latest version of Firefox (both stable and beta versions are available on the Android market). Try clearing the cache memory of your phone. Sometimes it happens when your devices memory is completely usedup or there are many background apps working. Cheers.