Is the problem that Firefox forgets that you unchecked the "Print headers and footers" box toward the bottom of the setup panel on the right side of the preview? I'm not sure why that would happen.
If you never want Firefox to add headers or footers on any printout:
Firefox stores a left, middle, and right header and footer for each of your individual printers. You can empty them all out through the following back door method:
(A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
More info on about:config: Configuration Editor for Firefox. Please keep in mind that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.
(B) In the search box in the page, type or paste print_header and pause while the list is filtered
Firefox should display one set of default values and then values for each printer you have used.
(C) Double-click each non-blank preference to display an editing field, delete out the current codes, then press Enter or click the blue check mark button to save the change.
(D) In the search box in the page, type or paste print_footer and pause while the list is filtered
Firefox should display one set of default values and then values for each printer you have used.
(E) Double-click each non-blank preference to display an editing field, delete out the current codes, then press Enter or click the blue check mark button to save the change.
