Just gave this a shot on my macOS (10.15.7) laptop, printing to a Canon iX6820 via direct cable. Mozregression led me to this [range](https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=ca5de0f3b7b94a5b28cdc3ff48ccb5a99e739991&tochange=3c6ac25bb5d5076576a99a81c12c654952297c3a), which includes [Bug 1670943: Black and White printing support for macOS](https://bugzilla.mozilla.org/show_bug.cgi?id=1670943). Prior to that patch, the drop-down to select color/monochrome is grayed out for macOS, so I couldn't confirm whether the monochrome print ever worked with my printer from the print UI before then.
Bug 1676191 Comment 4 Edit History
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Just gave this a shot on my macOS (10.15.7) laptop, printing to a Canon iX6820 via direct cable. Mozregression led me to this [range](https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=ca5de0f3b7b94a5b28cdc3ff48ccb5a99e739991&tochange=3c6ac25bb5d5076576a99a81c12c654952297c3a), which includes [Bug 1670943: Black and White printing support for macOS](https://bugzilla.mozilla.org/show_bug.cgi?id=1670943). Prior to that patch, the drop-down to select color/monochrome is grayed out for macOS, so I couldn't confirm whether the monochrome print ever worked with my printer from the print UI before then. Feel free to ping me if you need me to test a patch on macOS.