User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:74.0) Gecko/20100101 Firefox/74.0

Steps to reproduce:

Was browsing Twitter.com when I launched another application that required elevation to run. Firefox had been running for 15-30 minutes at this point without issue.

Actual results:

When the UAC prompt appeared for the other application, the Firefox window appeared corrupted, with black rectangles appearing over both the content and the window decorations. The black rectangles appear at somewhat regular intervals, as if they are tiles.
Recording below shows what happens when moving the mouse over the affected window. The problem applied to the entire window (all tabs) and didn't go away until the browser was restarted.

I have UAC configured at the low security setting, so showing the UAC prompt did not involve switching to the secure desktop.

Expected results:

No black rectangles should appear.

The video demonstrating what happens with the affected window is at: https://youtu.be/xoHXhJwWqto

Carey, have you seen this again?

No, I haven't seen it again.
I've been running the beta with gfx.webrender.all enabled since the blog post came out, and I've only seen this issue once.

Suspected fix has shipping with Firefox 75 (bug 1617083) - we will reopen if this is still an issue.