Firefox launches new browser when printing PDF files
I am on macOS Mojave and since the last Firefox update, I have been experiencing problems when printing a PDF attachment in my email. When I open the attachment, it opens in html which is fine. Then when I click print, a new tab opens with the printer settings, etc. but Firefox also launches a new browser, not just a new tab in the same browser. I have tried to change the print settings, reset the print settings, etc. but it has not helped. I can print the same attachment in another browser, such as chrome or safari without any issues.
The attached image shows my browser in the background with the pdf and printer window open in one tab and then the newly launched browser in the foreground which is blank.