This could be a problem with some of the fonts on your Mac computer like Times or Helvetica

See:

• /questions/1268528 when I type the number 9, the underscore symbol appears

Thanks. But the problem doesn't seem to be specific to a particular font, or to fonts installed on my machine. And it persisted when the first font I noticed it with, Times, was re-installed.

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/Tools/Page_Inspector/How_to/Edit_fonts

Thanks. I located an example of the problem I’ve been seeing, at luc.devroye.org

The italic word “Density” shows as “Densit”

Rules shows font: serif and bgcolor: ffffff Both lines are crossed out and accompanied by warning signs.

Fonts, under Fonts Used:

Apple Color Emoji Apple Symbols Hiragano Kaku Gothic Lucida Grande Times Times New Roman

What’s being used to display the problematic text might be Times or Times New Roman.

i've run into the same problem here. 'y' not displayed, 'x' rendered as quote. Pages display properly in Safari but not in Firefox 72.0.1 Example page: http://nevada-outback-gems.com/prospect/chris_prospect.htm In Inspect Element, the page is apparently specifying Comic Sans MS but is being rendered in Times bold. Times is selected in my Firefox preferences as the default font. Most pages render correctly. In my example page, selecting 'view page source', returns a display of the text and text is displayed correctly. i'll attempt to include a screen shot here of what i'm seeing.

Running Firefox 72.0.1 on a Macbook Pro 15", OS Sierra v. 10.12

I also have this problem (Mac version 72.0.1) - and have checked that it does NOT exist with Safari. I am appending two .png files of the same page - one taken from Firefox and the other from Safari. The poem is mine, by the way....