This bug is 14 years old. As I recall, this margin depends on the print.print_margin_* settings which can be set from about:config in Firefox. I long ago fixed this problem in my local copies of Firefox, but it may be that the default settings from Mozilla still aren't proper. I have all these four settings at 0.5 and have no problem anymore with printing margins. They're marked as "user set" in my Firefoxes so I'm not sure what the default settings are or were.
This bug is 14 years old. As I recall, this margin depends on the print.print_ margin_ * settings which can be set from about:config in Firefox. I long ago fixed this problem in my local copies of Firefox, but it may be that the default settings from Mozilla still aren't proper. I have all these four settings at 0.5 and have no problem anymore with printing margins. They're marked as "user set" in my Firefoxes so I'm not sure what the default settings are or were.