There is normally no "Print" button in a Thunderbird message window or message tab.
But you can print a message by left-clicking the "More" button and then selecting "Print..."
However when you do that you don't see a "Print Preview" so you don't know exactly what the print will look like. This can sometimes cause the resultant print to be too big or too small.
So, left-click the Thunderbird menu button (the three horizontal lines at top right), hover over "Options" (in Windows) or "Preferences" (in Linux) and then left-click "Toolbar Layout..."
The Customize Toolbar box will open - scroll down it to find the "Print" icon and drag it to drop it just above the Thunderbird menu button.
Left-click the "Done" button at the bottom of the Customize Toolbar box to close it.
Now you have a print button and a little down arrow that allows you to choose "Print..." or "Print Preview".
Go for "Print Preview" every time and sort out the "Scale" and "Page Setup" before you go for the "Print" button where you can choose the page range and number of copies.