Upon re-reading your observations, some further thoughts.
In Firefox, under Tools > Options > Tabs, there is a checkbox to warn you if opening more tabs will cause Firefox to slow down. Is that box checked?
Take a look at Task Manager > Performance to see how you're doing on CPU and memory utilization. If CPU utilization is high, check Processes to see what is grabbing cycles.
Is the power mangement on your system slowing the CPU because of overheating? If so, maybe a little vacuuming will help. Maybe someone forgot the thermal paste on the CPU-cooler interface.