Often I get messages that say 'turn off Mozilla Firefox' in order for software to update. ie. Adobe Flash Player.
But I don't know how to turn Mozilla Firefox.. off so I can update.
Simply close all open Firefox windows/tabs before running the update installer.
Note that you will have to SAVE the update installer, then close Firefox, navigate in Windows Explorer to wherever you saved the update, and double-click on it to run the update installer.