Firefox is rendering window and pages transparent and flickering
Solution 1:
Problem: Firefox show transparent window when set gfx.webrender.all to true in about:config.
Solution: Set it to false and restart the browser then all back to normal.
Source: Improve Firefox Performance on Linux by Turning WebRender On
Explaining: WebRender is the brand new Mozilla render developed in Rust lang that's replacing the old GECKO render developed in C++. However, it's for experimental use yet.
Solution 2:
Same problem here in Linux Mint 20.2.
In NVIDIA X Server settings, switching Enable FXAA to false in Antialiasing settings solved the issue.