Refusinik! I wonder why you're so keen on the old version though? I suspect you had some bad experiences with version 4, perhaps? That one wasn't their greatest, I have to admit.
They've made some serious improvements to FF since that version - especially security improvements and bugfixes. Why not try it out on another machine/partition/VM and see how it goes? Stick on all the add-ons you want and try it all out. Modern FF is fast and stable and renders very well. In short, it's the business.
If you must ask, yes I did have. It disabled AdBlock (which is something that makes the Internet tolerable to me) and some other plugins. Wasn't fluid and used more RAM (and I noticed this more than any other, because at any given time, I usually have some 30-odd tabs open). I also don't like the layout/GUI and how they messed with the settings you could changed. It's basically the same situation when I was forced to work with Office 2007, coming from Office 2003.
This is also my way of responding to the childish behavior Mozilla has taken towards Google's Chrome scheme.