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 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/docs/Tools/Page_Inspector/How_to/Edit_fonts

Done and Inspector says that it using Fira Sans and Arial, and Firefox's settings set to use default font. Opened additional settings and uncheked option to allow sites use it's own fonts and it did help.

Keep in mind that disabling "Allow pages to choose their own fonts, instead of your selections above"will cause issues with websites that use an iconic font to display special small images.

This is more likely a problem with the copies of these fonts that are installed on your computer and you need to check if there are updates available.