Have you tried Canon support? Assuming the Canon Mycamera Plugin you have installed for Firefox is related to that printer.

"Duplex" is usually done differently on Inkjet vs. Laserjet printers, which type of printer is involved? (With the limited number of printers I have worked with, inkjets usually involve the user flipping the paper for the 2nd side, where laserjets usually automatically re-feed the the paper for printing on the reverse side.)

What is "Phase 2"? Is that where you have to flip the printed sheets of paper to print the even numbered pages on the reverse side of the odd numbered pages?

Have you checked the Properties for that printer from within Firefox?
Many printer settings are saved separately in Firefox and need to be set from within Properties when viewed thru Firefox. You can't count on "system defaults" being used by Firefox.

This should have nothing to do with Canon. i have no canon devices in my system.

The printer is a hp Laserjet 4100 with a duplex addon installed, printing preferences are all defaults with duplex enabled.

Sometimes I print on a single page that already has one side printed (similar to your description of manual 2nd side printing for inkjets).

Printer settings from within Firefox are identical to the settings in IE or Word or any other program.

Even if it was possible, i do NOT want to have different printer settings just for Firefox. Firefox should be able to read system defaults and follow my preferences (they are all defaults anyway)

It seems to me this is a bug in Firefox

Sorry!

I just discovered that ALL my applications started printing on the opposite side of page! (compare to before)

I have no idea what has caused this as I have not changed any settings in the last year!

What I have noticed though, Firefox takes an extremely long time (50 seconds) to start printing (download to printer using Ethernet connected printer), even for a simple page with no graphics! everything else takes a couple of seconds to start.