After all the hype, I finally downloaded Firefox Quantum and the first thing i noticed, while opening GitHub, FOUC happened! I was like, OK, it might be an anomaly but happened again on multiple websites with loads of css & javascript.
If anybody else is wondering: "FOUC stands for Flash of Unstyled Content." <a href="https://webkit.org/blog/66/the-fouc-problem/" rel="nofollow">https://webkit.org/blog/66/the-fouc-problem/</a>