A Mozilla official said the bug poses very low risk to users. Mozilla doesn t plan to fix the bug, as there is little chance of it being exploited. URL obfuscation often is used by attackers to hide the true address of a malicious site that they re directing users to, typically as part of a phishing or drive-by download attack. iFrames are a favorite of attackers because of their ability to perform malicious actions in the background of a victim s Web session.
Source: https://threatpost.com/new-firefox-iframe-bug-bypasses-url-protections-081710/74339/