I know this is a little off, but looking through those extensions, almost all except maybe one, have been ported over to Chrome. I was an avid Firefox user years ago. They released version 3 that was sooo laggy when it first released I couldn't stand it.
Then I tried out Chrome at about version 6 and never looked back. I had to use Firefox on a clients machine the other day and I can't believe how slow it feels coming from Chrome. I'd at least try chrome before trying to track down a memory leak in Firefox.
I would also grab a copy of malware bytes and ensure your system if free from trojans and malware, browser slow down is normally the first place we will detect an issue with it. (Have I mentioned Chrome is entirely sandboxed and rated the most secure mainstream browser available?)
Best of luck!