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:
Mozilla/Components/compreg.dat
Mozilla/Components/xpti.dat
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.