> Firefox under Debian defaults to US_Letter after every single upgrade.

Create a user.js file in ~./mozilla/firefox/xxxxxxxxx.default which contains the desired printer paper size parameters you wish to "permanently" set to  A4. This user.js file is optional. If you have one, then whenever the application is started it will overwrite any settings in prefs.js with the corresponding settings from user.js. The user.js file should remain intact and unmodified through every debian update that includes upgrades that replace Mozilla Firefox files.

Note that for the folder ~./mozilla/firefox/xxxxxxxxx.default, the "xxxxxxxxx" is a randomly generated folder name that is unique for each computer.

see: http://kb.mozillazine.org/User.js_file...

About the user.js file

A user.js file can make certain preference settings more or less "permanent" in a specific profile, since you'll have to first delete or edit the user.js file to remove the entries before the preferences can be changed in the application. This has the advantage of locking in certain preference settings. A user.js file is also a way of documenting preference customizations and it makes it easier to transfer customized settings to another profile.

When you launch your Mozilla application, valid preferences you've added to the user.js file are automatically copied to the prefs.js file (located in the same profile folder) where all user-set preferences are stored. For this reason, you should make a backup copy of the prefs.js file before you create or edit the user.js file....