Mozilla Firefox provides option to automatically check for software updates. But if this option is disabled and you want manually check for updates and install them then you can use Mozilla Firefox Help menu’s check for updates option.
Click on Firefox Help menu
Select Check for Updates… from menu list. This would display the following dialog box
If any latest version is available then it would be displayed as shown below
Click on the Get the New Version button.
Once the downloading of latest Firefox is complete, the following Update Ready to Install will be displayed. Click on the Restart Firefox button.
The window below would be displayed if any open Firefox tab are kept open. Click Restart button to continue with the Firefox software update