There are some relatively minor changes in FF21
http://www.mozilla.org/en-US/firefox/21.0/releasenotes/ and if you do the upgrade it will take care of the previous version without you needing to do anything.
If you want to turn off automatic updating then go to the Options in the Tools menu, select the Advanced tab and then the Update section of that. There you have the option to have updates done automatically, or have it still check but ask whether you want it done, or leave it to you to do manually. The program version can be manually checked for updates by going to the Help menu and selecting the About Firefox option (in the pop-up window that results there is a Check For Updates button).