There are ways to format HTML messages that will frustrate Firefox every time. However, in most cases, you should be able to print, but you need to use the Outlook print formatted page. This is available on the "..." menu.

By the way, when sites pop up windows with a special purpose, they often remove features such as the menu bar, which makes it difficult to use the Preview feature. To make sure the orange Firefox button or classic menu bar (whichever you are using) is available in the popup window, you can change a setting in the about:config preferences editor.

Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste window_ and pause while the list is filtered

(3) Double-click the dom.disable_window_open_feature.menubar preference to flip it from false to true.

The meaning of this preference is hard to discern, but with the default setting of false, sites have the option to turn the menu bar off, and with the setting changed to true, sites are not able to turn it off.

If you are using the classic menu bar, on some sites that have tightly sized the window, this might cause content to disappear at the bottom of the pop-up. If you find that happening often, you might need to switch this setting back.