I likewise encountered this problem after upgrading from Ubuntu 16.04 to
18.04. Duplex printing works fine from other applications. I currently
have Firefox 62.0.3 installed.
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Duplex printing in Firefox broke after upgrading from 16.04 to 18.04. I
did not altered CUPS configuration during this upgrade.
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https://bugs.launchpad.net/bugs/1736680
Title:
fixefox
Status changed to 'Confirmed' because the bug affects multiple users.
** Changed in: firefox (Ubuntu)
Status: New => Confirmed
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https://bugs.launchpad.net/bugs/1736680
Title: