You are talking about in the computers files, yes? Open the file browser. On the Menu bar, select Tools. Now select Folder Options. In the new window, select View. Look for Hide extensions for known file types and turn it off. Now press OK on the bottom of the window.

Firefox has been my browser for the past 20+ years. After selecting Tools in the Menu bar, there is no Folder Options choice in that menu. What am I missing? Thanks.

That's not a Firefox preference, it's part of Windows.

File browser = Windows Explorer - not to be confused with Internet Explorer

Control Panel > Folder Options -> View --> Advanced Settings

Turn off Hide extensions for known file type

Second reply. I had been unable to find Folder Options until I entered it in the Search space above Start. I could then proceed from there as you instructed. Thanks.