Printing in Firefox (MGA6, Plasma) just gives an empty page (just header and footer)
It maybe related to : "Firefox Addon disabled (old xpi certificates)" because it happened about the same time.
It turns out 2 parameters should be reset to the usual values. This solves it.
Solution :
ensure MGA6 is updated ie. Firefox 60.6.2esr
In the Firefox printing preview window :
page size : somehow the values for A4 paper was wrong , selecting another size and then back to A4 returned the correct values
scale : was set to custom and I changed it back to "shrink to fit".
Hopefully a clean install of F 60.6.2 will not have this issue.