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
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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