Youtube theater mode search box bug

When I login to my google account / youtube account I get this bug when I hover over the search box on youtube (in theater mode). After that, whatever time I watch a video, this white box will always be there. Even if I restart the browser or logout. The only thing that gets rid of this is apparently to delete this file: Firefox\Data\profile\cookies.sqlite. When I delete that I’m automatically logged out and the white box is gone. But as soon as I login again the white box comes back when I first hover over the search box.

I have tried a reinstall of Firefox, login with a different account, but with no success. The only thing that seems to help is to change the browser.

Anybody know how I can get rid of this? It have always worked fine before…