Firefox dropped support for Mac 10.11 as of Firefox 79 (since you're using 78 I guess you're aware of that):

https://www.mozilla.org/en-US/firefox/79.0/system-requirements/

Unfortunately issues like this are very unlikely to get fixed in old ESR versions of Firefox which are only getting security fixes. Sorry that's not very helpful. :-(

Thanks for the info...unfortunately it doesn't help. At one time you could use a different font on a Mac but I guess it doesn't work that way anymore.

You can right-click and select "Inspect Element" to open the builtin 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

See these threads about similar font issues:

• /questions/1278708 Text of website https://apod.nasa.gov/apod/ is missing the charakters x, y, z but only in lower case where x is not simply missing but replaced by "
• /questions/1292109 Fonts replaced with question marks in boxes
• /questions/1268528 when I type the number 9, the underscore symbol appears (long dash)