Firefox can be a huge memory hog. I have seen it use as much as 3GB of memory on my machine. If you only have 4GB, you're in trouble. Upgrade to 8GB if possible. Switch to Chrome or Midori. I have been running AdBlock for years and am quite satisfied with it.
You might also have limitations in the video display capabilities of your video adapter and/or drivers.
SSD will only help with browsing when it comes to caching pages. If you are going to spring for a SSD, pay attention to sustained read and write speeds. Not all SSDs are created equal. IMHO Samsung Pro SSDs are the best bang for the buck.