I made the mistake installing Firefox 100 only to learn that I cannot print page numbers without also printing full headers and footers. I've been using Firefox years and have been able to print page number/of total number of page at bottom right of each page. This is not the first time Firefox has screwed up in introducing updates -- extremely disappointing.
It is more complicated to manage after Firefox 85 when the old Page Setup dialog was replaced by the new combined preview/setup overlay. You still have complete control over your left, center, and right header and footer fields through about:config. Here's how:
Left, Center, and Right Header and Footer Preferences
Headers and Footers are split into three parts each of which can contain free-form text and placeholders that Firefox replaces with values computed at print time. The placeholders are:
Here is how you can update your header and footer content for particular printers or all printers:
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
(2) In the search box in the page, type or paste print_h and pause while the list is filtered -- you'll see something like the following (the exact number of custom preferences depends on how many different printers you have used):
The following three preferences are the default, but any printer-specific value overrides them, allowing you to customize individual printers:
To edit a default setting, double-click in the implicit second column (wide area to the right of the preference names before the pencil button) or click the edit button. After making changes, press Enter to save your change, or click the blue checkmark button.
To modify a setting for a particular printer (for example, print.printer_Mozilla_Save_to_PDF.print_headerleft), you have two choices:
(A) To have Firefox use the default setting, delete the current custom setting using the trash can at the right end of the row for the custom setting.
(B) Edit the custom setting as desired.
(3) In the search box in the page, type or paste print_f and pause while the list is filtered
As with headers, the following three preferences are the default, but any printer-specific value overrides them, allowing you to customize individual printers:
Edits work the same way for footers as they do for headers.
Can you get it working the way you need?
P.S. I wouldn't mind if you or someone else posts a proposal in the "Ideas" section to bring header/footer customization to the new setup panel.