Issues with letter combinations like "fi" could be a problem with ligatures. You can check in Font Book for font issues like corrupted and duplicate fonts. Note that you shouldn't disable "Allow pages to choose their own fonts, instead of your selections above" as this will cause issues with iconic fonts used by webpages to display small icons (you may see text labels instead of icons). 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/ *https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/examine_and_edit_html/ 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/

The Fonts tab shows that the font used is Helvetica but I can't see a Rules tab. I am not a programmer so am looking for a simple response that an average user can understand. Thank you.