How do I know the version of my FireFox browser?

Open FireFox help menu (arrow 1 on the image) and select "Help" menu item (arrow 2 on the image) on the image:

Open 'Help' FireFox menu

In the new menu select on "About FireFox" menu item:

Open 'About' FireFox dialog

Then you can see the version of your FireFox browser:

Checking FireFox version