Firefox still uses OpenGL to draw the window to the screen and also uses transparency in those windows, unfortunately neither of those things play nice with the

Firefox still uses OpenGL to draw the window to the screen and also uses transparency in those windows, unfortunately neither of those things play nice with the scaled windows in MacOS. They are in the process of converting their code to use Core Animation instead of OpenGL to draw to the screen, which will help tremendously with the power consumption issue, so help is on the way. As a temporary workaround until all those changes land, you can have Firefox opt out of using scaled windows on a retina Mac by getting info on the Firefox app and then checking ”Open in Low Resolution” on the get info panel. Transparency in Firefox can be turned off by going to about:config, and setting gfx.compositor.glcontext.opaque to true.