Right, so i've discovered something just an hour ago. I'll post this here hoping someoen else with the same problem can see if this solution works for them;
Firefox has a config setting labelled "layout.frame_rate". This is set to "-1" by default, which should correspond to the refresh rate of the monitor in terms of vsync values. However, this doesn't seem to work. So i believe firefox was getting capped to 60hz / 60fps.
So go to about
:config in the browser, and then set "layout.frame_rate" to 120. Being the 120hz refresh rate i am using with the lightboost feature. This works!
Firefox is now as crystal clear as it is on Windows10. Which i've confirmed by viewing this ghosting test:
https://www.testufo.com/ghosting , all 3 panes are moving without any motion blur.
So it appears that firefox is somehow not correctly setting this value, and it needs to be manually adjusted for monitors higher than 60hz.
EDIT: I've posted a bunch of information here relating to set up this monitor on Linux; Along with the EDID.bin i am using; If you're using this monitor on Linux and looking to fix the problems hi-lighted in this thread, i recommend having a look here for some solutions.
https://github.com/Jigoku/Acer-GN246HL-linux