Make sure you are not blocking content.

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop also see https://blog.mozilla.org/security/2021/03/23/introducing-smartblock/

https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

Sorry, the problem still exists even when Enhanced Tracking protection and SmartBlock are disabled.

What about cookies and cache?

Please check out: https://support.mozilla.org/en-US/questions/1364032#answer-1475084

No, the problem still exists after fully clearing cookies and cache.

Make sure you allow pages to choose their own fonts.

• Settings -> General: Fonts & Colors -> 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".

Make sure you allow pages to choose their own fonts.

• Settings -> General: Fonts & Colors -> 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".

I never touched that font setting before and I checked it is on by default.

As I mentioned, this issue even happens in Troubleshoot mode with no addons running, so it is not a uBlock issue. (block remote fonts is also off by default)

Delete browsing, search and download history on Firefox {web link}

Remove a single website from your history {web link}

Open the History Manager <Control> (Mac=<Command>) <Shift> H. In the search bar, enter the name of the site. Right-click on one of the listings and select Forget About This Site. This should remove all information, including any site settings And Passwords.

Delete browsing, search and download history on Firefox {web link} Remove a single website from your history {web link} Open the History Manager <Control> (Mac=<Command>) <Shift> H. In the search bar, enter the name of the site. Right-click on one of the listings and select Forget About This Site. This should remove all information, including any site settings And Passwords.

I have already cleared my complete cache and cookies, and I never save passwords on Firefox.

You see little boxes with the hex code of the character if you do not have a font installed that covers this Unicode range. Special (small) icons are usually supplied by an iconic font that is downloaded from the server via @font-face CSS rules.

You can check in the Rules tab in the right panel in the Inspector what font-family is used for selected text (i.e. right-click a little box with the hex code -> Inspect). 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

You can check gfx.downloadable_fonts.enabled on the about:config page and make sure it is set to true (if necessary double-click the line to toggle its value to true).

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

You can check the Web Console for messages about fonts that fail to download.

• "3-bar" menu button -> Web Developer or Tools -> Browser Tools -> Web Developer Tools
• https://developer.mozilla.org/en-US/docs/Tools/Web_Console

Thank you for that info on the fonts! The console said it couldn't access fonts.gstatic.com, so I added it to my whitelist on my PiHole and that fixed it after another cache refresh. :)