Affected versions

• Fx91.0b5
• Fx92.0a1

Affected platforms

• Windows 10
• Ubuntu 20
• macOS

Steps to reproduce

1. Launch Firefox.
2. Go to about:sessionrestore page.
3. Hit CTRL+P to open the print modal.

Expected result

• The new printing modal opens up so the user can print the page.

Actual result

• Nothing happens.

Regression range

• This looks like a recent regression as the printing modal works fine in the latest RC.

Additional notes

• If the printing modal is opened from the hamburger menu, the preview is stuck in an infinite loading animation.

I've managed to reproduce the issue on Windows 10x64, and then investigated for the change that made this behavior possible:
2021-07-26T10:22:48.367000: DEBUG : Found commit message:
Bug 1666247 - Part 3: Disable tab modal print on about:sessionrestore r=emalysz

Differential Revision: https://phabricator.services.mozilla.com/D116877

2021-07-26T10:22:48.367000: DEBUG : Did not find a branch, checking all integration branches
2021-07-26T10:22:48.369000: INFO : The bisection is done.
2021-07-26T10:22:48.371000: INFO : Stopped

This seems like it should be fixed on the last nightly / beta via bug 1714639 shouldn't it?

I confirm that this bug is fixed on the latest Nightly 92.0a1 (2021-07-26) .

Duping this over per comment #2. :-)