Firefox Paper Size
It appears that no matter what the system papersize is set to, and no matter what cups says is the default paper size, firefox, in its infinite wisdom locks it to being letter - this would probably be fine if I was in the US, but I'm not, and I want it to be A4...
So, the stages to go through to get it to do the right thing appear to be:
Goto about:config
Filter on print_paper_size
Change any bold (i.e. set) entries that are wrong to A4
This seems like something that should be much easier set, apparently using Page Setup only affects the current tab/window you are in and doesn't change the default, which is just odd.