We're real humans here.

Firefox 73 worked normally? Did the update change anything else?

While this may not be as pretty as you had before, could you review your default fonts and change them to TrueType fonts for testing? Here's how:

Open the Preferences page:

• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Windows: "3-bar" menu button (or Tools menu) > Options
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

Scroll down to the Language and Appearance section, and click the Advanced... button. This should open the Fonts dialog.

(1) Select the Latin character set

Near the top there is a character set selector. Latin is used for pages in English.

(2) Change the fonts to basic TrueType fonts

Your computer may have a diverse range of fonts, from the ones supplied with the system to ones you've added individually or through other software packages. Firefox may work most reliably with the built-in system fonts.

• Serif - Change from Times or Times Roman (or other serif font) to Times New Roman

• Sans-serif - Change from Helvetica (or other sans-serif font) to Arial

• Monospace - Change from Courier (or other fixed-pitch font) to Courier New

(3) Repeat #1 and #2 with the "Other Writing Systems" character set

(4) Click OK to save changes

Any difference on the problem sites?

Screenshot for reference:

Note that this can be caused by using a third-party font manager that installs fonts in a location that Firefox can't access due to sandbox security restrictions.

This possibly means that Firefox isn't able to access the requested font and falls back to a 'last resort' font that shows all glyphs as a '?' or a 'A'.

You can check for sandbox security issues when Firefox has a problem to access fonts installed locally.

• https://wiki.mozilla.org/Security/Sandbox#Content_Levels