Firefox 3.5 can be configured to check for updates automatically using the Tools -> Options. The settings can be accessed by clicking on Tools menu followed by options -> Advanced tab -> Update sub tab.
The Automatic check for updates are available for Firefox, Add-ons and Search Engines. This can be enabled by ticking the corresponding check box. There is also options available for displaying the updates that had already been installed. This can be done by clicking Show Update History button.