Please do the following.

1. Right click a blank part of the Tab bar and then click "Customize" at the bottom.
   
   
2. Find the print button in the panel which opens and drag it on to the toolbar (along with anything else you think might be useful).
   
   
3. Similarly, you can remove buttons, or toolbars you don't want by dragging them into that panel. They'll remain them just in case you change your mind and want them back again sometime.
   
   
4. While the panel remains open, you can also re-arrange the layout by dragging elements like buttons and toolbars around.
   
   
5. Click "Done" once you're satisfied with the result.
   
   

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Thanks.

• { Ctrl + P } from the keyboard works = like with most other Windows programs
• File > Print = when the Menu Bar is showing
• Firefox button > Print -> Print = when the Firefox button is showing and the Menu Bar is hidden