If an attacker is able to alter specific about:config values...

Vulnerability Description

If an attacker is able to alter specific about:config values (for example malware running on the user's computer), the Devtools remote debugging feature could have been enabled in a way that was unnoticable to the user. This would have allowed a remote attacker (able to make a direct network connection to the victim) to monitor the user's browsing activity and (plaintext) network traffic. This was addressed by providing a visual cue when Devtools has an open network socket. This vulnerability affects Firefox < 87.

References

• https://www.mozilla.org/security/advisories/mfsa2021-10/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1659129