It seems the problem of memory usage and CPU usage is at least partially caused by ForecastFox. Of course, Firefox and ForecastFox are blaming each other for the problem.

Now, with the release of 2.0.0.2 of Firefox and 0.9.5.2 of ForecastFox, the CPU problem seems to be gone. It may take a few days of usage before I notice any different regarding the memory issue, but now, my Firefox hardly registers in CPU utilisation...