I am guessing one problem with this bug is that it really needs upstream attention, and there's not enough information to tell whether the problem lies with Firefox or GTK itself. The problem may well even lie with KDE.
Manu, you say the problem happens in Synaptic and Inkscape, as well? That would lead me to believe that there is a problem with GTK. Perhaps there is an parameter in the API that disables autocompletion, and GIMP uses it, so it is not affected (or vice versa).