This is a new rendering backend for Firefox that’s not yet on by default. Presumably there are some edge cases where it makes things worse or causes some instability, but so far I have not experienced anything bad. On the contrary, without it, I and some other people get terrible flickering in Firefox on Wayland. With it enabled, not only is the flickering gone, but scrolling performance becomes buttery smooth and CPU usage decreases noticeably on both Wayland and X11, resulting in increased battery life! Win-win-win.
To turn it on, visit the
about:config page in Firefox, search for “gfx.webrender.all”, and set it to true. That’s all there is to it!