The right way to solve your problem is to go to Firefox Settings/Language and Apperance/Fonts, then click on "Advanced".

There you can set what fonts should be used by Firefox to display each script/language, including Chinese, Japanese and other CJK variants.

If you do not configure this, then it is indeed unpredictable which fonts will be used by Firefox to render the Web pages, unless it can match exactly a font requested by the page.

I've reproduced issues with Unifont's glyphs being invisible in firefox and chrome (at different times, on what seems like certain versions), with much confusion. There are a few issues on the nixpkgs issue tracker about this, including one about Noto Color Emoji doing the same thing.

I love fonts...