Using Firefox nightly in Debian "testing" on a Thinkpad P50 with NVIDIA Quadro M2000M and "nvidia" driver version 390.87, with "layers.acceleration.force-enabled" set to "true": Steps to reproduce: 1) Open youtube.com in Nightly. 2) Switch to a different user. 3) Switch back. 4) Scroll the YouTube home page. Results: Some or all of the chrome *and* content layers in the window become 100% transparent. Background windows and the desktop background are visible, instead of the content of the Firefox window. Some layers may remain visible and scrollable. The window is still responsive (though you can't see most of what it's doing). I believe this behavior may have started after the fix for bug 1492580 landed. See also bug 1489354.

Thanks for the report. We should try to get this fixed, but disabling acceleration is the official solution for now.

Just for additional info: In my case i have several Firefox windows. After switching users and changing back window paints black and tab content (as well as tabbar) is no longer accessible. However, another windows is working fine and i can switch to it. If i go to about:support i can see failures [GFX1-]: [OPENGL] Failed to init compositor with reason: FEATURE_FAILURE_OPENGL_CREATE_CONTEXT in Graphics section. Now i can push Trigger Device Reset button and "black windows" returns to fully usable state. In terminal (stdout) i then (but it seems not before i do device reset) get: WebGL(0x7fae581e6000)::ForceLoseContext WebGL(0x7fae581e6800)::ForceLoseContext Unflushed glGetGraphicsResetStatus: 0x92bb [GFX1-]: [OPENGL] Failed to init compositor with reason: FEATURE_FAILURE_OPENGL_CREATE_CONTEXT [GFX1-]: Compositors might be mixed (2,1) [GFX1-]: Compositors might be mixed (1,2)