This specific problem is due to installing a version of the Helvetica font (very commonly used on the web) that either Windows or Firefox can't display. To confirm, all you have to do is paste the following text in your Firefox address bar and hit enter:
<font face="Helvetica" size="25">abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ</font>
<font face="Helvetica Neue" size="25">abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ</font>
If it displays as weird symbols, then you can be sure that's your problem.
To fix it, all you have to do is go to %WINDIR%\Fonts and delete all the fonts named Helvetica.
If for some reason, Windows doesn't allow you to, you have to go to the registry, navigate to
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts and then delete all the entries that have Helvetica on their name.
If you are using Windows 7 64-bit, you also have to delete the Helvetica entries from the following registry key.
After this, you only need to reboot your computer and the fonts will be displayed correctly again. At least that fixed it for me.