Are you downloading Firefox from the official mozilla.org source? If so, it does not have a rootkit, this is likely a false positive by your anti-virus (it happens from time to time).

If you are concerned, please scan for malware using the tools at Troubleshoot Firefox issues caused by malware

Some antivirus clients are still occasionally flagging the stub installer for something ever since they existed since Firefox 18.0 release yet have no issues with the full setup. Never been proven that any Release for Windows is indeed infected with whatever was claimed.

Full setup can be found at https://www.mozilla.org/firefox/all/