Fix blurry firefox under wayland

I have an HDPI screen with fractional scaling of 1.5, and firefox was blurry under wayland sway. I tried all combinaison of environment variable but it was either zoomed in too much or still blurry. I finally found the culprit, which is a firefox setting. In about:config change the following setting: widget.wayland.fractional-scale.enabled must be set to true