By default, Firefox is set to update automatically but you can always do a manual update. A manual update will still let Firefox download an update but it won't install it until you restart Firefox. Here's how to set it up:

Click the menu button , click Help and select About Firefox.
The About Mozilla Firefox window will open. Firefox will begin checking for updates and downloading them automatically.
When the download is complete, click Restart to update Firefox.

Important: If the update didn't start, didn't complete or there was some other problem, you can go to the Systems & Languages page to download and install the latest version of Firefox for your system and language or you can use this download link (see How to download and install Firefox on Windows for more information). Stay safe: Download only from the official Mozilla links above to avoid unwanted programs like malware.

Update settings can be changed in Firefox Options . Click the menu button New Fx Menu