This could indicate a corrupted font, or an incompatibility with your graphics card drivers. To work around the latter, could you try disabling Firefox's use of hardware acceleration of graphics?

You usually need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

orange Firefox button (or Tools menu) > Options > Advanced

On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available"

If you restart Firefox, is the issue resolved (or at least improved)?

I turned off hardware acceleration and the italic bold problem is still there.

This can be a problem with the font that is used to display the text.

You can select this italic/bold text and use "Inspect Element" to open the built-in inspector.
If you click the Fonts tab in the pane on the right then you can see which font is used.

You can do a font test to see if you can identify corrupted font(s).

• http://browserspy.dk/fonts-flash.php?detail=1

You can try different default fonts and temporarily disable website fonts to test the selected default font.

• Tools > Options > Content : Fonts & Colors > Advanced
• [ ] "Allow pages to choose their own fonts, instead of my selections above"

Unchecking the "allow pages to choose their own fonts" solved the problem. Thank you.

I would still advice to check which font is causing your problem and try to reinstall or delete the not working font. Otherwise you still have a (problematic) not working font installed.