You can check in Font Book for font issues like corrupted and duplicate fonts. You can right-click and select "Inspect" to open the built-in Inspector with this element selected. *https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/open_the_inspector/ You can check in the <b>Rules</b> tab in the right panel in the Inspector what font-family is used for selected text. You can check in the <b>Fonts</b> 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://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/edit_fonts/