How to upgrade Firefox in OS X
The screenshots below are taken from Mac OS X, but the process is the same on Windows.
Simply opening the "About Firefox" interface is enough to trigger Firefox to check for updates and download them if there are any found.
Click the "Firefox" menu
This will reveal a menu with lots of options. One of them will be named "About Firefox".
Click "About Firefox"
You will be taken to the About page for Firefox.
Firefox will automatically check for updates
The About screen may look different to the one in the screenshot above. Version numbers shown here are examples and may change depending on what is considered the latest version...
Firefox tells you "Checking for updates..." while it checks for a newer version.
If Firefox finds updates, it will prompt you to install them
If a newer version is found, you will see a button prompting you to update to the latest version.
When you are ready, you can click this button to trigger the update. The new version will start to download.
Firefox will download the latest version
When the download is finished and Firefox has applied the changes, it will prompt you to restart the browser.
Click the "Restart Firefox to Update" button to restart it.
Firefox has now been updated
If you view the "About Firefox" screen again, you will see that Firefox is now running the latest version.