Now updates are important to plug any security hole but sometimes an update can slowdown an application then there is always a risk software crash for an update that isn't properly tested. I have always found Automatic Update of any application discomforting and feels like giving away too much control. Look at the case of iPhone update that rendered the unlocked ones like a door stop !
So I don't update until there is a security issue that needs to be resolved and see if that update causes any problems before I decided to update. So I have disabled Automatic updates for pretty much all my applications.
Here is how to Stop automatic update for Windows and Firefox
Right click on my computer > Properties to access System properties, you can also access it from the Control Panel.
Go to Automatic Update and choose Turn Off automatic Updates.
In Firefox go to tools > Options > Advanced > Update
Under the section when Update are found click on "Ask me what I want to do"
To know more about the Update in firefox Visit Release Note Page for the details about that update and the bugs which it will fix