Tavis Ormandy just tweeted about a bug in the Chrome version of the WebEx Extension that allows RCE from any page that includes a magic string in its URL. https://twitter.com/taviso/status/823642226093760512 The tweet links to the Project Zero bug which is now public and contains the script your page could need to include to execute shell commands on the client's machine. Searching DXR it appears we have at least 5 add-ons that contain this magic string. Have not searched to see if there are other WebEx add-ons that use a different Firefox-specific url but that would seem unlikely (if they're the same then they don't need to UA sniff to serve the right page).
in our repo I find https://dxr.mozilla.org/addons/source/addons/735601/ id: firstname.lastname@example.org ver: 1.0.1 https://dxr.mozilla.org/addons/source/addons/735588 id: email@example.com ver: 1.0.0 https://dxr.mozilla.org/addons/source/addons/735573 id: firstname.lastname@example.org ver: 1.0.0 https://dxr.mozilla.org/addons/source/addons/730787 id: email@example.com ver: 1.0.1 https://dxr.mozilla.org/addons/source/addons/728001 id: firstname.lastname@example.org ver: 1.0.0
The first one looks like it is the Cisco WebEx Extension on AMO, judging by the id number thing. https://addons.mozilla.org/en-US/firefox/addon/cisco-webex-extension/?src=search
Thanks for the quick turnaround.