That CPU and RAM consumption is likely due to extensions (AdBlock Plus being the most common hog). Unless you're running a really old version, Firefox is usually smaller than Chrome these days.
Yup ABP is killer. I'm also unwilling to upgrade to Australis at the moment, as I can't find a Mac-compatible theme that isn't painful to use with it. (Hate, hate, hate monochrome themes -- color coding the buttons may not be trendy but it improves usability, and has for the 20 years I've been using web browsers.) And Firebug has some major performance improvements with FF31 and newer.
Actually almost forgot about this, but I tried watching a video on FF recently, and Chrome used about 5 percent of my CPU (with about 10 other tabs open), and a lot of extensions, too, while on Firefox I had just a few add-ons (some of the same as on Chrome), and only that video running. It used 15 percent of my CPU.
