firefox html problem

[wander]

Minor one here,

Basically for some reason when I go to save an mp4 video it now comes up as a firefox html document (instead of an mp4) and I cannot change it to saving as an mp4. Also once saved, it saves as a webm instead and is basically unwatchable.

It's a minor thing as it's not like I dl many videos, though I hear some people with firefox are having an issue like this with pdfs and those I do dl quite a few of through drivethrurpg for my rpg sourcebooks, which I pay money for.

I'm sure it's some setting has been knocked somewhere as it didn't always do this, it just randomly changed yesterday. Any help would be great, cheers. :)