This was already found in #392344, though I didn't backtrace as Firefox was not compiled with debug info. That bug exists since libpango1.0-0 1.14.5-1 at least.
Here's my version info (based on firefox 1.5.dfsg+1.5.0 dependencies, running Etch) ii debianutils 2.17 Miscellaneous utilities specific to Debian ii fontconfig 2.4.1-2 generic font configuration library - support ii libatk1.0-0 1.12.2-1 The ATK accessibility toolkit ii libc6 2.3.6.ds1-4 GNU C Library: Shared libraries ii libcairo2 1.2.4-1 The Cairo 2D vector graphics library ii libfontconfig1 2.4.1-2 generic font configuration library - runtime ii libfreetype6 2.2.1-5 FreeType 2 font engine, shared library files ii libgcc1 4.1.1-13 GCC support library ii libglib2.0-0 2.12.3-2 The GLib library of C routines ii libgtk2.0-0 2.8.20-2 The GTK+ graphical user interface library ii libjpeg62 6b-13 The Independent JPEG Group's JPEG runtime li ii libpango1.0-0 1.14.5-1 Layout and rendering of internationalized te ii libpng12-0 1.2.8rel-5.2 PNG library - runtime ii libstdc++6 4.1.1-13 The GNU Standard C++ Library v3 ii libx11-6 1.0.0-9 X11 client-side library ii libxft2 2.1.8.2-8 FreeType-based font drawing library for X ii libxinerama1 1.0.1-4.1 X11 Xinerama extension library ii libxp6 1.0.0.xsf1-1 X Printing Extension (Xprint) client library ii libxt6 1.0.2-2 X11 toolkit intrinsics library ii psmisc 22.3-1 Utilities that use the proc filesystem ii zlib1g 1.2.3-13 compression library - runtime