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I don't know anything about how Firefox is packaged for snap, but snap's "sandboxing" might interfere with getting all fonts.
You might want to try using Firefox without snap (which has some other benefits, especially around startup time) or adding ~/.local/share/fonts (which is where fonts are supposed to be installed for users) to some sort of allowlist.
If the fonts are installed as systemwide snap or flatpak applications may not be able to see them. Since they are allowed only to the user directories by default.
You can copy the fonts to user directory
cp /usr/share/fonts/your-font ~/.local/share/fonts/
Seems like this is a bug: https://askubuntu.com/questions/1427695/how-to-make-snap-firefox-use-locally-installed-fonts
Thanks for the comments. Based on them I found https://github.com/snapcrafters/gimp/issues/21 which suggested copying the font to ~/.fonts/ (which didn't work) and to /usr/local/share/fonts/ which fixed it - Epiphany can now see the font, and I can now set it as default in Firefox (tho Firefox is ignoring it in a page's CSS).