How do you fix missing characters from uncommon scripts? I already have Noto Sans Newa font installed on my computer
I have tried a restart.
I'm pretty sure LibreWolf just uses default fonts and ignores the ones installed on your system. It's a measure against tracking. A website can make your browser render different fonts and then e.g. read out the size of the text block and then, depending on whether you have the font installed or not, that size will be different (because the actual font characters will have a slightly different size from the missing character symbol or from characters from other fonts) and they can use that to see what fonts are installed on your system, which can be used to track you.
Is there a way to change this behaviour?
Don't think so. I think it's part of Firefox's advanced tracking protection and you can only turn that off completely but that would defeat the whole point of using LibreWolf. You could search up if there's a way to explicitely add fonts to LibreWolf/Firefox but I don't expect there to be one.