User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:58.0) Gecko/20100101 Firefox/58.0 Build ID: 20171005220204 Steps to reproduce: 1. Install Firefox 57+ 2. Enable dark theme in right-click > Inspect > Settings icon (cogs) in top right of the inspect panel > Themes > [x] Dark 3. Visit github.com Actual results: Github's favicon on the tab is pretty much invisible. I can't recall this being a problem with Firefox <= 56, so this might be a regression with the new theme. Expected results: Firefox should probably detect logos below a certain darkness and surround them by a white box, since most favicons seem to be designed with a default bright theme in mind. Alternatively, just always put a white background behind logos in the dark theme (this is probably what Firefox <= 56 used to do? I forgot)