User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:79.0) Gecko/20100101 Firefox/79.0

Steps to reproduce:

Invoke the print dialogue to print a message (via any means - CTRL-P in the message window, message window More>Print.., menu File>Print...)

Actual results:

Blank (plain white) window with no UI elements and empty name (blank title bar) opens. The window can be resized, closed, etc.

The error console throws the following errors every time the attempt is made:

Uncaught ReferenceError: XPCOMUtils is not defined
<anonymous> chrome://global/content/printUtils.js:61
printUtils.js:61:1
<anonymous> chrome://global/content/printUtils.js:61

Uncaught TypeError: PrintUtils is undefined
PrintEngineCreateGlobals chrome://messenger/content/msgPrintEngine.js:30
OnLoadPrintEngine chrome://messenger/content/msgPrintEngine.js:21
onload chrome://messenger/content/msgPrintEngine.xhtml:1
msgPrintEngine.js:30:3
PrintEngineCreateGlobals chrome://messenger/content/msgPrintEngine.js:30
OnLoadPrintEngine chrome://messenger/content/msgPrintEngine.js:21
onload chrome://messenger/content/msgPrintEngine.xhtml:1

Expected results:

Standard print dialogue should appear.

Note, this worked fine up to the last 79.x (beta-channel) - stopped working after upgrading to 80.0b1 (Linux, beta-channel)