Bad Unicode Character Display on JIRA Web Interface using Firefox

Symptoms

• There are some missing icon or image in the User Interface (UI) and showing something like bad unicode character
• Other browser (Firefox) on other machines works fine
• Problem only occurs on a particular browser and happen to all JIRA including Atlassian Support System and Atlassian JIRA Public Bug Tracking
• The user interface is showing something similar like the following screenshot
• 

Diagnosis

• Switching browser character encoding has no effect on the issue
• Private browsing does not work

Cause

• It is because the following is set in prefs.js which can be set in Options > Advanced Font Setting

user_pref("browser.display.use_document_fonts", 0);

Resolution

1. Go to Options > Content tab > Advanced Font settings 
2. Tick or enable "Allow pages to choose their own fonts, instead of my selections above"