There are a lot of UTF-8 symbols on Firefox and Safari that render using a system font rather than "Noto Emoji," which causes many characters to render as colored when they should render as black and white. Since this also seems to be a problem on the official Google Fonts Noto Emoji page (many symbols render using the system font on this page), it's unclear if there's any simple solution to this problem.
My interim patch for these problems has been to use older and simpler UTF-8 characters and check if they render on the Google Fonts page before including them.
There are a lot of UTF-8 symbols on Firefox and Safari that render using a system font rather than "Noto Emoji," which causes many characters to render as colored when they should render as black and white. Since this also seems to be a problem on the official Google Fonts Noto Emoji page (many symbols render using the system font on this page), it's unclear if there's any simple solution to this problem.