Firefox shows garbage instead of jpg (only when embedded in iframe)

[LCID Fire]

Binary package hint: firefox-3.5

Instead of showing the actual jpg firefox displays more or less random garbage when attemting to get a close look at e.g. Wikipedia images (previously found via Google image search).
This is only reproductible when the image is displayed withing an iframe. And best of all - there are 3 users on this system and only 1 is facing this problem - the other 2 cannot reproduce this behaviour (it works using the same steps). I'm baffled since I cannot imagine which system component is user centered and also changes the system deep enough to cause such a graphic glitch.

ProblemType: Bug
Architecture: amd64
Date: Wed Feb 24 17:48:48 2010
DistroRelease: Ubuntu 9.10
Package: firefox 3.5.8+build1+nobinonly-0ubuntu0.9.10.1
PackageArchitecture: all
ProcEnviron:
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-19.56-generic
SourcePackage: firefox-3.5
Uname: Linux 2.6.31-19-generic x86_64
XsessionErrors:
 (gnome-settings-daemon:3435): GLib-CRITICAL **: g_propagate_error: assertion `src != NULL' failed
 (gnome-settings-daemon:3435): GLib-CRITICAL **: g_propagate_error: assertion `src != NULL' failed
 (polkit-gnome-authentication-agent-1:3544): GLib-CRITICAL **: g_once_init_leave: assertion `initialization_value != 0' failed
 (nautilus:3527): Eel-CRITICAL **: eel_preferences_get_boolean: assertion `preferences_is_initialized ()' failed
 (firefox:3628): GLib-WARNING **: g_set_prgname() called multiple times