[Firefox] Scrollbars, picture-in-picture #285

Scrollbars

Scrollbars on firefox have a moderate amount of customization:

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/scrollbar-width

The color of the track can be set to transparent, and the width can be none (which is necessary for the taskbar). Along with that, the width can also be set to thin, which is probably what we'll want to try to match the style we get on browsers like Chrome and Brave.

Picture-in-picture

Along with scrollbars, users report seeing a picture-in-picture button when viewing videos in ArcOS. I'm not sure if this can be fixed, Google searches were inconclusive, but hopefully there's something we can do about this. One of the options we might want to consider is to simply disable pointer events on the media element of the media player, as I found some reports of people getting rid of the PIP button with that method. Here's the PIP button in question: