I have installed Firefox 15 in our Citrix environment but would like to apply setting from Group Polices
As far as I know, regular Firefox does not include support for group policy. You can specify preferences in a file named mozilla.cfg, but this is not a registry-based solution.
Are you considering deploying the "ESR" version of Firefox? More info:
FrontMotion distributes a custom build of Firefox that supports ADM. I haven't tried it myself: