Certain problems can be solved by performing a Clean reinstall. Please follow these steps one by one and tell us if it helped:

1. Download the latest Firefox from http://www.mozilla.com
2. Close all Firefox windows by going to File > Exit, confirm all upcoming messages.
3. Now, uninstall Firefox by following the steps mentioned in the Uninstall Firefox.

IMPORTANT: The uninstaller has the option to remove your personal data and settings. Make sure that you do not check this option.

Once this is done:

1. Go to Start > Run, enter %PROGRAMFILES% in the textbox and press Enter. An Explorer window should open.
2. In the Explorer window, see if you can find a Mozilla Firefox folder. If you do, right-click it and select Delete. Confirm that you want to move the folder to the trash.

Now, go ahead and reinstall Firefox:

1. Double-click the downloaded installation file and go through the steps of the installation wizard.
2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

Please report back to see if this helped you!

I've tried this for my mac, but it hasn't resolved the issue

You can also do a check for corrupted and duplicate fonts and other font issues:

• http://www.thexlab.com/faqs/multipleappsquit.html - Font Book 2.0 Help: Checking for damaged fonts
• http://www.creativetechs.com/iq/garbled_fonts_troubleshooting_guide.html