I'm running a machine with the same specs as you and I don't ever experience Firefox bogging down as you are describing. I do believe Firefox is more heavy-weight than Chrome, but nothing to the point where I have to consider updating my OS or running an SSD. I set my browser to delete everything when I close and launch again. This helps to keep registry and temp files from building up. Perhaps this might help?

I've never been able to get firefox to play well for me from the start (for me 2008). I searched for something faster and I found opera and used that.

As is mentioned earlier, firefox does not appear to take advantage of multi-threading. I take this to mean that it's performance would be the same on a quad core cpu or a single core cpu of the same speed. There's much more to it than I know for sure, but certainly feels like it has to share resources with itself as well as anything else running. I tried it out a couple months ago and then went back to Opera/chrome. Many of the same plugins are available in chrome that are offered for firefox.

Firebug is one of the worst extensions I've seen in terms of performance. Uninstall it and see if there are changes.

Especially since devtools are native in Fx now.

If you don't have an SSD, upgrading to one would make a huge performance difference. You can also try Pale Moon and see if that works better for you.

You should certainly also upgrade to Windows 8.1 at least, if not Windows 10. There aren't any compatibility differences that I've ever noticed, and there are absolutely performance benefits.