That is likely nor an encoding problem but a problem with a specific font like an Arial font.

You can check in Font Book for font issues like corrupted and duplicate fonts.

This can be caused by a problem with fonts you have installed locally and that might be corrupted or are missing variants. You will have to identify and remove corrupted font(s).

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 Rules tab in the right panel in the Inspector what font-family is used for selected text. You can check in the Fonts 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/

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).

Thanks for your answer. I let Font book check for corrupt fonts or duplicates but the search came up empty handed, no duplicates or critical errors. However, after the scan the problem disappeared, as if the scan itself fixed the problem. Conclusion, you're absolutely right. It clearly had to do with one/some of my installed fonts, and simply scanning them fixed the problem somehow :) Again, thank you, and merry Christmas!