I tested it and it works, but be careful, there is no source code I can see and to modify headers you need full permissions on all pages. You can also enable/disable it from Firefox add-on manager, as needed.

One such way that social media accounts are exploited is when users are enticed to download malicious browser extensions that request read and write permissions on all websites. These users are not aware that later on, typically a week or so after being installed, the extensions will then download some background Javascript malware from its command and control site to run on the user's browser.