icedove/iceweasel gui wrong font size & display switching

Hi, I have some annoying gui font sizes problems with icedove 3.0 (i.e. thunderbird) and iceweasel 4.0 (i.e. firefox) under Trinity on Debian Squeeze. I use a laptop (1280×800) with a docking station and an external 19" screen (1280*1024). When I plug the laptop on its docking station and change display settings to use the 19" screen, everything resizes correctly (the desktop, trinity apps, etc.) except iceweasel and icedove that begin to use very huge fonts sizes in gui display (menus, toolbars, etc.). When I set display back to the laptop screen, the issue is still there. Recently, I upgraded to iceweasel 4.0 and I remember that I didn't have this issue with iceweasel 3.5.x. So I guess this might be linked to a wrong integration of gtk (libgtk2.0-0, versions above 2.10) into Trinity. Are there too many new features in gtk that are not well handled by gtk-qt-engine-trinity? You'll a screenshot of the situation here: http://users.klvb-larve.org/nico/bugs/icedove-wrong-font-size.png Advice are welcome! Nicolas PS: more details on the icedove & iceweasel versions: I got iceweasel 4.0 (4.0.1-1~bpo60+1) thanks to this backport repository available from mozilla.debian.net: http://mozilla.debian.net/ squeeze-backports/iceweasel-4.0 i386 Packages I use the Debian stable version of icedove: 3.0.11-1+squeeze2