This can happen if you block remote fonts that are downloaded from internet or otherwise block website fonts.

Make sure you allow pages to choose their own fonts.

• Settings -> General -> Language and Appearance -> Fonts -> Advanced
  [X] "Allow pages to choose their own fonts, instead of your selections above"

If you use uBlock Origin make sure not to "block remote fonts".

This can happen if you block remote fonts that are downloaded from internet or otherwise block website fonts. Make sure you allow pages to choose their own fonts.

• Settings -> General -> Language and Appearance -> Fonts -> Advanced
  [X] "Allow pages to choose their own fonts, instead of your selections above"

If you use uBlock Origin make sure not to "block remote fonts".

Thanks, cor-el.

I infact use uBlock Origin, but it is not blocking remote fonts. As well as in settings, pages are allowed to choose their own fonts. I tried blocking the fonts in uBlock Origin, refreshing the website, and unblocking again, but nothing changed. I also tried to unmark allowing pages to choose their own fonts - that helped in these same websites, but other websites messed up. It looks like that these certain pages changed their fonts, but it's weird to me - looks like they have nothing in common and it's some kind of a bug

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

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