Have you tried restarting Firefox with add ons disabled? Just in case one of them is causing you problems.
If it all seems ok, then you can activate them one at a time, to find out the guilty party.
My Firefox is fine, the only adds ons/plug ins I use are AdBlock Plus, English Dictionary and the pop-up blocker is enabled. Never have too many tabs open at a time (my limit is about 15) and restart Firefox about every hour.
Firefox did at one time use excessive memory the longer it ran, which denied memory to other processes and hence you would get a 'slow' down.
I would rather have Firefox than Chrome, mainly because I have no love of Google.