Created attachment 202492 [details]
taskbar

Just found that there is a difference in
/usr/lib/firefox/chrome/icons/

3.0b4 contains:
default16.png
default32.png
default48.png

while 2.0.0.x contains just one
default.xpm (in 48x48)

I need to check if it's a build issue or what changed upstream.

Ok, they dropped the XPMs and introduced PNGs but only up to 48x48 pixels.
I'm not quite sure what resolution of the icon the KDE taskbar is using in your screenshot. I would expect 48x48 not to look like crap in your taskbar.
Probably we are using the wrong sized icon in the first place?

https://bugzilla.mozilla.org/show_bug.cgi?id=413607

I think there is a wrong icon used for the taskbar. 

In my impression the one used for the ALT-TAB-window looks a lot better and they are about the same size.

Hmm, we just hand over a list of icons to Gtk's gtk_window_set_icon_list().
I have no idea how that is handled with KWin (or whatever it's called with KDE4).

Felix, do you think you can modify the icons in firefox/chrome/icons/default so that we actually can see which one is used for what? Probably just creating correct sized colored boxes. That would tell us which icon is used. I'm not absolutely sure it's a Firefox bug but tend to point to Gtk or KDE because of comment #5

hmm, just take back my comment that it might be a KDE thing since I see the behaviour in WindowMaker that the very small icon is used in the dock.

Ok i colored all my icons with the following result:
the taskbar uses /usr/lib/firefox/chrome/icons/default/default16.png
but ALT-TAB does use /usr/lib/firefox/chrome/icons/default/default32.png

After talking to vuntz we believe it's most proably KDE's panel which has to choose the correct icon from the list. So reassigning for checking that.

Fixed upstream (r786632).