You can specify the font with code in userContent.css.

• http://kb.mozillazine.org/userContent.css
• https://developer.mozilla.org/en/CSS/@document
• https://developer.mozilla.org/Web/CSS/font-family
• https://developer.mozilla.org/Web/CSS/font

@-moz-document domain(<enter domain>){
html, body, body * { font-family: "<font name>" !important; }
}

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

• Create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• Use a plain text editor like Notepad to create a (new) userContent.css file in the chrome folder (file name is case sensitive)
• Paste the code in the userContent.css file in the editor window
• Make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userContent.css.
  Otherwise Windows may add a hidden .txt file extension and you end up with a not working userContent.css.txt file

Thanx cor-el... I should have been more specific. I'm already utilizing a userContent.css file... A handful of sites do not display certain icons correctly if they are not allowed to use their own fonts. I would like to keep 'Allow pages to use their own fonts...' UNchecked in options but force a specific domain to behave as if it is checked. This way I can view them as is determined by the site meanwhile the rest of the web is using the font(s) specified in userContent.css. I hope this is clearer...

What code do you use in the userContent.css file? Is that site specific with @-moz-document domain(){} rules or generic CSS rules for all websites?

I'm using a specific font for all websites. This works just fine on 99% of the sites I visit... However, discogs.com (for example) will not display some icons correctly unless 'Allow pages to use their own fonts...' is checked.