Eventually I had to remove Ghostery, completely uninstall Firefox, delete all vestiges of Firefox from the computer, and then reinstall it. The lagging finally quit but I'm fearful of putting Ghostery back on because of the same thing happening again.
New versions of Firefox create as many problems as it resolves.
I have read of some problems with v23 or whatever that latest is.
I primarily run v15, because later versions conflicted with Adobe Flash and would not download PDF files. Many of the extensions don’t work with later versions and many developers gave up trying to keep up.
Mozilla has taken a good browser and basically wrecked it with their rapid release insanity.
Ghostery started using 50% of the CPU on one of my machines. Maybe the same problem. Removed it and went back to Noscript for a somewhat similar purpose. Noscript has additional benefit of freeing memory and improving performance because unwanted scripts are not even loaded.
Usually clearing out the directory where personal Firefox settings are stored fixes these problems but is too much hassle — passwords, etc.