The new install (137 is new) maybe involves a new profile. Search this forum for new profiles and how to transfer data. Printers are their own world, especially with drivers. Can you verify the brothers driver as updated?

Try this. see screenshots https://support.mozilla.org/en-US/kb/fix-printing-problems-firefox

jonzn4SUSE - thank you for the reply but I had already tried Troubleshoot fix as previously stated and it did not work, hence me asking about a config fix. Basically, where in about:config can I set Firefox to only got to the WIN11 system printer. When I am in about:config I do not seem to be able to make any changes - has v137 got some sort of lockout on changes? I have has this problem with previous versions of Firefox and been able to fix. It would be far simpler and more user friendly if Mozilla developed code which allowed users to assign any printer they needed rather than the current approach. The premise that Firefox reverts to the last printer used dynamically (ie PDF) is less than satisfactory. I have been using Firefox for over ten years and do not want to be driven to using another browser. Thank you.

What about the system settings for default printer? Let boot my win11 laptop and see.

Here's what I see in Windows. see screenshot I assume you've tweaked these settings? My printer is currently not connected, but you see my default.

Thank you, system printer (Brother MFC-L3750 series Printer is set as default and prints from Chrome and other software ok. Have set the above name in about:config and is visible when Ctrl+P as second option under Save to PDF. Brother option greys out when selected for Ctrl+P. This is my problem.

Sounds like it's time for a Firefox refresh. https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings