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Mac:

Select the Firefox menu, then "About Firefox."
firefox menu with about firefox selected underneath

In the display window, the version will appear below the word Firefox:

version number will appear under the word firefox

Windows (later versions of FireFox):

Select Firefox in the top left-hand corner, then hover over the "Help" option to open the drop-down list and select "About FireFox."

Information Technology Support > Determining the version of FireFox > FireFox version.PNG

In the Window that pops up, the version will appear below the word FireFox:

version number will appear under the word firefox