If the problem is only on one site look at the character encoding used. Changing it for a one off on that page may solve the problem. You may find that using the auto detect, or a specific one for your language works most or all of the time.

See also

• Websites look wrong or appear differently than they should and note that the encoding option is adjacent to the page style option.

You may have a problem with the font that is used to display this text.

• font-family: "Bitstream Vera Sans","Lucida Grande","Trebuchet MS",sans-serif;

You can use this extension to see which fonts are used for text that is selected (right-click: "Show fonts in selection").

• fontinfo: https://addons.mozilla.org/firefox/addon/fontinfo/

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"