Firefox slow as HEL even after boosting RAM from 4GB to 16GB

The title says it all: Firefox is the only piece of software that shows zero sign of improvement in its performance after quadrupling the amount of RAM installed on my 64-bit desktop computer. All other applications show a significant boost in performance. Yet, among all the applications I use, Firefox is the one that most needed this speed boost. To say that this is a major disapointment is an understatement. FFS, Mozilla devs!