Thank you for getting back to us here on the Community page, @-Alex.
I'm here to share with you the steps on how to achieve this on the Firefox browser.
Usually, this happens when the PDF and Portable Document Format is not set up to Preview in Firefox. For now, you have to change the settings from the Applications section.
Here's how:
- From your Firefox browser, go to the hamburger icon or three horizontal lines at the upper-right corner of the page.

- Select Options.

- Scroll down to the Applications section, then select Preview in Firefox from the drop-down.

Once completed, you'll be able to print your invoices seamlessly.
As always, feel free to read the details from this link to learn more about printing invoices in QuickBooks Online: How to email or print an invoice.
Also, feel free to read the topics from our help articles in case you need references while working with QuickBooks in the future.
Post again if you have any other questions, @-Alex. I'm a few clicks away to help you out. Keep safe!