It is very annoying that Firefox does not remember the page format once I changed it in "File->Page Setup...". If I would use the german version of Firefox maybe defaults would have been set to A4 anyway - however in the english version (at least on Ubuntu Linux) the default is Letter.
Here is how to change it (to A4 in my case):
- In the location bar enter: "about:config"
- Read carefully and confirm that you are going to be careful
- Be careful with the following steps!
- In the search bar enter "paper"
- Look for properties ending with paper_size and paper_name.
- For me it was sufficient to change "print.postscript.paper_size" and "print.print_paper_name" to "A4".
- There might be some additional properties ending with paper_size and paper_name for each of your available printers. I didn't touch them and it worked anyway - YMMV.
Related posts: Locale configuration on Ubuntu, OpenOffice and LibreOffice starts slow, Default Paper Size in Open Office, Normal.dot in OpenOffice or LibreOffice.