We seem to have the same issue introduced with Ubuntu 14.04LTS:-
Error: stackunderflow, Command: pop
but only when printing from some websites with Firefox 36 (and before that 35).

Printers affected: Brother HL-5240, and Brother HL-6050, using the recommended 'Brother <model> BR-Script3' drivers.

Bug IS Reproducible in 1404LTS
------------------------------
Load one of the following index/home pages in Firefox 36.0 and try to print it:-
https://www.gov.uk  - almost any page on this site seems to fail
http://www.lloydsbank.com/
http://www.bostonglobe.com/news/bigpicture
http://www.harrington-fabric-and-lace.co.uk

All these sites print OK from Firefox 35 or 36 in 1204LTS with the recommended BR-Script3 drivers, and also in 1404LTS with the  Foomatic/ljet4 driver as detailed below.
Some of these sites are important services which we and many others need to print pages from. Most other websites print OK.
We don't tend to print such large or complex pages with other apps, so that might be why we only notice it with Firefox.


We tried resetting all Firefox's printer settings by editing prefs.js and deleting all lines containing 'user_pref("print.print_' but it made no difference. 

To avoid this bug we have had to change from the recommended 'Brother ... BR-Script3' drivers, to: Generic > PCL6/XL > Foomatic/ljet4. This driver gives good results for the HL-5240, but poorer image quality (greys/half-tones) for the HL-6050.

1404LTS suffers this bug
===================
Kernel: 3.13.0-46-generic x86_64 (64 bit)
Desktop: Unity and Xfce
Firefox: 36.0+build2-0ubuntu0.14.04.4
Using recommended drivers:-
  Brother HL-5240 BR-Script3 v1.03
  Brother HL-6050 BR-Script3 v1.00
cups  1.7.2-0ubuntu1.5
cups-browsed  1.0.52-0ubuntu1.2
cups-filters  1.0.52-0ubuntu1.2
foomatic-db-engine  not installed
foomatic-filters  not installed
foomatic-db-compressed-ppds  20140410-0ubuntu1
hplip  3.14.3-0ubuntu3.2
libcupsfilters1  1.0.52-0ubuntu1.2
$ locale
LANG=en_GB.UTF-8
LANGUAGE=en_GB:en
LC_CTYPE="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_COLLATE="en_GB.UTF-8"
LC_MONETARY="en_GB.UTF-8"
LC_MESSAGES="en_GB.UTF-8"
LC_PAPER="en_GB.UTF-8"
LC_NAME="en_GB.UTF-8"
LC_ADDRESS="en_GB.UTF-8"
LC_TELEPHONE="en_GB.UTF-8"
LC_MEASUREMENT="en_GB.UTF-8"
LC_IDENTIFICATION="en_GB.UTF-8"
LC_ALL=

1204LTS does NOT have this bug
==========================
a) on the machine above (now running 1404LTS) when it had 1204LTS and Firefox 35
b) on a different machine with Firefox 35 or 36, currently as follows:-
Firefox 36.0+build2-0ubuntu0.12.04.5
Using recommended drivers:-
  Brother HL-5240 BR-Script3 v1.03 (same as 1404LTS)
  Brother HL-6050 BR-Script3 v1.00 (same as 1404LTS)
cups  1.5.3-0ubuntu8.6
cups-filters  1.0.18-0ubuntu0.2
foomatic-db-compressed-ppds  20120322-0ubuntu1
foomatic-db-engine  4.0.8-2ubuntu1
foomatic-filters  4.0.16-0ubuntu0.2
hplip  3.12.2-1ubuntu3.4
libcupsfilters1  1.0.18-0ubuntu0.2
$ locale
LANG=en_GB.UTF-8
LANGUAGE=en_GB:en
LC_CTYPE="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_COLLATE="en_GB.UTF-8"
LC_MONETARY="en_GB.UTF-8"
LC_MESSAGES="en_GB.UTF-8"
LC_PAPER="en_GB.UTF-8"
LC_NAME="en_GB.UTF-8"
LC_ADDRESS="en_GB.UTF-8"
LC_TELEPHONE="en_GB.UTF-8"
LC_MEASUREMENT="en_GB.UTF-8"
LC_IDENTIFICATION="en_GB.UTF-8"
LC_ALL=