If you have enabled automatic updates in Firefox 5 then using the Show Updates History you can check list of Firefox updates that have been installed. For example if you want to check whether the recently released security update is installed then you can use this feature.
Click the Tools menu and select Options from the list of available menus
In the Options window, navigate to Advanced Tab and then to the Update sub tab.
To see the list of Updates installed for your Firefox, click the Show Update History button. This would launch the Update History window which would show the installed updates as shown below.
If you want to know more about any particular update then click on the Details link available adjacent to that Update.