To be able to display headers information in the PageInfo tab of Mozilla or Firefox the nsHeaderInfo component must be registered.
This component should be found in the file Mozilla/Components/nsHeaderInfo.js.

Registration should have been done by the LiveHTTPHeaders installation.
However this sometimes does not work.
Follow these steps to enable Mozilla or Firefox to find and register the component:

These two files should be recreated automatically by Mozilla or Firefox when it is next started.

Note: if you installed LiveHTTPHeaders in you profile, there is a copy of the nsHeadersInfo.js file in your profile/components directory. You need to copy it in Mozilla/Components directory first and follow the directive above after.