Windows – Why do all the browsers default to the “Impact” font

You can actually find the fonts in a Windows 8 install cd under the /Windows/Fonts folder. Just copy those over to the Fonts folder on your computer.

More instructions (with other options) can be found at the following online article: http://www.askvg.com/fix-unknown-characters-or-vertical-rectangles-are-showing-in-place-of-metro-icons-in-windows-8-start-screen-and-login-screen/

If you know which fonts you want to delete, the solution that worked for me was to forcefully delete the files from the command prompt in Windows advanced settings. To do so, do the following:

1. Hit Win + C to open the "Charms menu"
2. Click Settings
3. Click Change PC Settings at the bottom of the pop-out
4. Click General
5. At the bottom, under Advanced startup, click "Restart now". WARNING: this will close all running programs and prepare to restart, so make sure you've saved everything you need.
6. Go to Troubleshoot > Advanced Options > Command Prompt
7. A command prompt will pop-up. You won't be able to change directories ("cd") into the Fonts directory (c:\Windows\Fonts) because the drive isn't mounted at this point, so you will have to delete the files directly. You can easily write a Batch file that deletes all the files, which will be easier if you're deleting a lot of files. I will list the two options below.
8. Once you have either entered the individual commands to delete the fonts, or executed the batch file, simply type "exit" to reboot your computer.
9. Verify that the files were deleted by going to the Fonts folder (c:\Windows\Fonts) and checking for the files there.
10. Celebrate the liberation of your system from those unwanted fonts.

Individually:

Type del "c:\Windows\Fonts\[font name here]", replacing "[font name here]" with the font filename (note: you can auto-complete the name with the tab button).

Batch File:

This needs to be done before going into the command prompt. Open up a text editor like notepad or notepad++, and enter the following (replacing [font name] with the font that you want to delete). Save the file as "c:\delete_fonts.bat"

@echo off

del "c:\Windows\Fonts\[font name]"  
del "c:\Windows\Fonts\[font name]"  
del "c:\Windows\Fonts\[font name]"  
del "c:\Windows\Fonts\[font name]"  
del "c:\Windows\Fonts\[font name]"  

Enter that line for as many fonts as you need to delete. Save the file again, and then start from step 1 again. Once you get to step 7, simply enter the following command at the command prompt to execute the batch script that you created to delete the fonts: "c:\delete_fonts.bat".

Well I ended up searching for the string font in about:config and reset all user-set options. It was a grind but I'm a cheap labor anyway ;)

But I'm sure there are better methods to do this. Perhaps someone with more experience on Firefox internal could enlighten me on how to do this programmatically.