The error Firefox is already running usually indicates that you must remove lock files before you can launch Firefox. To resolve this problem:
If you are working inside the HMDC RCE navigate to Applications → RCE Utilities → Clear Firefox Locks or Clear Thunderbird Locks. Then, try launching Firefox or Thunderbird again.
If you are not working inside the HMDC RCE, type the following command via a terminal to delete the Firefox lockfiles:
rm -f ~/.mozilla/firefox/*.default/{lock,.parentlock}