Support for enablePrivilege has been removed in Firefox (Bug 546848), so it is no longer possible to get special elevated privileges by using this method.
Bug 546848 - Remove support for enablePrivilege
If a web page is using this then they will have to use other ways get what they want like providing a Firefox extension.
This issue can also be caused by an extension that isn't working properly and that tries to inject code on a web page.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do not click the Reset button on the Safe mode start window or otherwise make changes.