Make sure that for the characters you want to see, the font you use supports it. Google Noto is compatible with most unicode standard emojis. Fonts in linux are stored in $HOME/.fonts, or ~/.fonts. Once the font is installed, restart firefox and change to that font in preferences.

You can right-click and select "Inspect Element" to open the builtin Inspector with this element selected.

• https://developer.mozilla.org/en-US/Tools/Page_Inspector/How_to/Examine_and_edit_HTML

You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Font tab in the right panel in the Inspector what font is actually used because Firefox might be using a different font than specified by the website.

• https://developer.mozilla.org/en-US/Tools/Page_Inspector/How_to/Edit_fonts