I've been using Firefox for the last 10 years or so and two things I have noticed are:
1. Installing the Adware option for Firefox helps with it's overall data usage and,
2. Over time Firefox seems to use more and more memory which slows it down and I always have Task Manager open in a window I can see and watch the memory usage for Firefox. W/O the adware the memory used to start off at around 900K and grow to over 2 to 2.5 million. I learned to periodically use Task Manager to end just the Firefox Process tree that resets the memory usage and hence restores the speed for Firefox.
I'm now with 1 and 2 starting off at around 600K and slowly say over 4 to 6 hrs grows to a little over 1 million at which time I end the Firefox process tree via Task Manager and restart Firefox to reset the memory usage back down to the 600K or so level. BTW I'm running Win7 Professional on an I5 processor with 8GB of memory with the OP sys on a SSD.
Larry