Windows 10
TB version 91*
Print opens and does not offer any control over the contents of the various parts of the header and footer.
It offers to print as is or not print.

It does not offer to choose what gets printed and where. Some examples:
eg: Footer - left by default has eg: 1 0f 1
How do I set this to be central ?
eg: Header right displays location of file storage
How do I choose to remove that?

Requires reinstating full control over headers and footers as in version 78* - control over various parts of headers and footers and what is displayed.

We won't be adding back detailed customization of header/footer. (But IIRC some of it can be set by hidden prefs.)
I agree with the Firefox developers, those were not very useful customizations.

I agree with the Firefox developers, those were not very useful customizations.

If Firefox developers do not understand the importance of being able to customise the content and positioning of headers and footer then perhaps there is a bigger problem.

One topic - Page numbering.
Being able to put page numbering in a central location or 'outer reading edge' rather than use default positioned on the bottom left is important.

If using harvard: Page numbers must be centered throughout the manuscript at the top or bottom.
People doing eg: degrees and who need to include eg: emails need full control over page numbering.

If using generally it is usual to position top or bottom on outer edge for lengthy documentation where several pages are involved - although centrally is common practise.
Thus positioning may depend upon what language is being used.
Reading left to right would be top or bottom on right side.
Reading right to left would be top or bottom on left side.

This bug should be reopened as it's importance is underestimated.

Another example of how Headers & Footers can be used which may not have been considered in the latest design.

An email can also be issued as a Task with job number and a person assigned.
This information is obviously not going to show in the actual print of email content.
The obvious location for this information was using the headers and footers.
eg: header center could be job number and person assigned header right
This could easily be done on a per print basis via the Page Setup Headers & Footers customisation.

Why do the developers think an improvement would be the following:
File > Print to check current headers
Cancel Print
Access Preferences > General
Access Config Editor
Edit preferences
Back to folder, select email and then File > Print

This really is not an ideal workaround because developers do not understand how headers and footers can used for various purposes.
Another reason why this bug should be reopened as it's importance is underestimated.

It would seem I'm probably going to need to write up an Article specifically for those using version 91* to explain how people can customise their headers and footers via the preferences in the Config Editor should they need to alter page numbering or anything else.

It really should not be a necessary thing to do, but if the developers are unable or unwilling to fix this problem then perhaps there is no option.
So before I spend time doing it - please confirm this removal of customisation - a common theme developing in Thunderbird - is the way it is going to be in future.

I'd agree with Anje that this a pretty painful loss of customisation UI. It's not every day, but for when it's needed (like for official documents), being able to fine-tune headers and footers matters. It's weird that the cryptic header/footer prefs are still used but no longer accessible via UI.
I understand that this has been dropped by Firefox and per mkmelin's decision in comment 1, Thunderbird doesn't want to spend resources on adding it back (and the maintenance costs of our own flavor would be too high as this is shared code).
So the odds are stacked against a revival, but it might be worth finding the bug where Firefox dropped this and to check the rationale there. You could also file a bug against Firefox to reintroduce UI for this, which would then also become available in Thunderbird.
Having documentation on those hidden prefs would be good if you'd be willing to write that up, Anje. Sorry for the inconvenience.
Prefs are easy to find by searching for header and footer respectively.

Another way of editing printouts (after the print was generated, but before it is printed) is using tools like FinePrint, which can add custom text, stamps, and page numbering across pages from different applications. It's a must-have tool for many other reasons, too.

Could someone please add these 3 key words to this bug?

ux-control User experience principle: users should always feel like they are in control of their software. (This principle is often the nemesis of

ux-interruption, especially in cases where developers assume users want more control than they actually want). Yes Search 259

ux-discovery User experience principle: users should be able to discover functionality and information by visually exploring the interface, they should not be forced to recall information from memory. (This is often the nemesis of ux-minimalism since additional visible items diminish the relative visibility of other items being displayed). Yes Search 203
ux-efficiency User experience principle: interfaces should be as efficient as possible, minimizing the complexity of actions and the overall time to complete a task.

Thomas brings up a good point that for people wanting special formatting there are tools out there specialized in exactly that.

At the very least the page numbering should by default be footer center in Firefox and Thunderbird
You would only have it on the bottom left if you read from right to left eg: arabic
The rational for putting it bottom left is perplexing. It's very unusual to set the default as bottom left as it would cause an issue for what I'm presuming would be a greater majority of users who read left to right.
Edit:
I did locate bug 1663436 on Toolkit Printing and asked for page numbering in footer to be centralised.