Having a similar issue, except mine is stuck on paper instead of A4.

Found a way to configure this:

In the address bar type "about:config" Promise to be careful (and be!) Enter "print.postscript.paper_size" in the filter box and press enter Double click the entry named "print.postscript.paper_size" Enter "paper" in the dialog box and press OK. (I changed mine to A4.) After this I restarted firefox to check that the change were permanent, and it was :)

I completely uninstalled Firefox, rebooted, and reinstalled. This has corrected the paper size issue noted above. However, it still has not corrected an issue for viewing PDF's. The PDF plug-in will work when initially installed, but when Firefox is closed and restarted, an error message appears.

I got stuck with Legal size paper. I found a couple of things helped my problem.

One: in Page Setup, Format and Options - turn off scale to fit page width

Two: Print, Properties - click Advanced Paper Selection and select the proper paper size. Then OK until you get back to print.

Option one from the previous post finally helped me change my default 'Letter' to 'A4' paper size type.

After faffing around for days changing the config (as helpfully described by other posts/forums) and various other settings, the simplest option of just turning off the 'shrink to fit Page Width' worked.

Thanks!

In "mine" dutch firefox 3.6.12 there is no option to change paper-size see "about:config" Promise to be careful (and be!) Enter "print.postscript.paper_size" gives a empty screen "paper_size" gives a few results

What about the numbers 4259872, 4784247 and what is the meaning of paper_size, paper_size_type , paper_size_unit

1. {Ctrl + P}
2. Make the correct printer is selected in Printer - Name.
3. Hit the Properties button.
4. Select the correct paper size there.

Those 7 digit paper size preference numbers don't look correct - looks like a b0rked setting to me, unless that is a "custom size" that is "User defined".