DaftPansy

A silly workaround is to not fullscreen the browser, but to leave it windowed slightly. Then it'll fullscreen properly when you fullscreen the video.
 
I'm unaware of any real fix; I never really looked into it.

Something to note about the dt770 pro, even though they are closed back, they do leak sound. Especially if listening to louder music. They don't leak as much as open back, but enough to where my gf can hear what I'm listening to if we're in the same room. You might want to test them out with someone else and see what you think. 
 
They are extremely comfy though and have a great sound signature (imo).
 
I have the 80ohm version and have zero trouble powering them with an old ipod, phone, or computer. 

It all comes down to personal preference.
 
I wouldn't wear my dt770s for public transportation or at work because everyone near me would hear. But for at home use, they are perfect. And they're the most comfortable headphones I've ever put on. The velour ear pads also might be why the sound leaks, but the comfort is unmatched. 
 
Between the 80ohm and 250ohm, I would choose the 80ohm every time because you don't need an amp to drive and the little extra bass. But i could see someone who plays fps games using the 250ohm because they have slightly more pronounced highs. 80ohm definitely wins for music. 
 
Oh, I should mention the cord is hella long. So you'll need to keep it coiled up for mobile use.