I have same problem, but is not only in firefox. Any text dragging results slowdown and kwin turns off desktop effects.
It is KDE 4.2.3, Ubuntu 9.04 64-bit (Kubuntu added via synaptic), desktop effects turned on. Graphics drivers is 180 series (nvidia's propriatary installed in hardware drivers applet)
I have same problem, but is not only in firefox. Any text dragging results slowdown and kwin turns off desktop effects.
It is KDE 4.2.3, Ubuntu 9.04 64-bit (Kubuntu added via synaptic), desktop effects turned on. Graphics drivers is 180 series (nvidia's propriatary installed in hardware drivers applet)
Hardware is GF 8800GT, Q9650 CPU, 8igigs of RAM