Hi, please : Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

This looks like a text-decoration CSS style rule (text-decoration: underline overline).

• https://developer.mozilla.org/en-US/docs/Web/CSS/text-decoration

Do you possibly use a userContent.css in the chrome folder in the Firefox profile folder?

You can try to use the Inspector to see if you can find the origin of this rule.

You can right-click and select "Inspect Element" to open the builtin Inspector with this element selected.

• https://developer.mozilla.org/en/Tools/Page_Inspector

Hi,

I tried Pkshadow's solution first, but restarting in Safe Mode had no effect.

I tried cor-el's solution next. I discovered that these particular Design Patterns book html pages do not use .css; they're probably about 20 years old or older. I looked in the Firefox chrome folder, but didn't find any "text-decoration" css rules.

The pages I'm having trouble with are local html files; however, I do see web html pages with this problem.

I can read the Design Patterns book html pages just fine in Safari, so I'll use Safari as a fallback.

Thank you, Don