Affected versions

• Firefox 79.0a1 (BuildId:20200618094105)
• Firefox 78.0b8 (BuildId:20200616235426)
• Firefox 77.0.1 (BuilId:20200602222727)
• Firefox 68.9.0esr (BuildId:20200527211442)

Affected platforms

• Windows 10 64bit
• Ubuntu 18.04 64bit

Unaffected platform

• macOS 10.14

Steps to reproduce

1. Launch Firefox.
2. Access the following link
3. Open print preview via the “Hamburger” menu.
4. Click the print option from the print preview and initiate the print job.
5. While the print job is in progress click the Landscape, Portrait or the Simplify page option.

Expected result

• Firefox UI should not break.

Actual result

• The Firefox UI breaks.

Regression Range

• I don’t think that this is a regression since this behavior is reproducible on older builds from 2016 as well.

Notes

• Clicking the print option (from the print preview) while the UI is broken doesn’t do anything and the following error message is thrown inside the browser console:
  TypeError: aBrowser.messageManager is null printUtils.js:135:5
• Clicking the “Close” button from the print preview fixes the broken UI.
• For further information regarding this issue please observe the attached screencast.