I did the upgrade on my main Mac. I think I'll leave the old version on my other iMac and PC.
I can't say it's a big deal - Mainly because I use Chrome for some things, Safari for others, and I also use Opera and even Camino from time to time. Firefox is my "main" browser but I run that "locked down" with NoScript and Ghostery. I only visit Google+, LinkedIn and Facebook with Chrome. Almost nothing else. I use Safari when I load a page in Firefox that has a bunch of stuff that NoScript and Ghostery block (notably embedded videos).
My only complaint on the Mac version of Firefox that I don't like is they have a very, very small line between tabs which makes it harder to distinguish where the tab starts and ends. But, since the tabs are essentially icons with a brief text following I just aim for the icon.
All in all it's just another UI change to get used to. I'm sure there are some "significant under the hood" changes, but few people care about that aspect.
As to speed - Well, many times it depends upon the site you're visiting anyway. I haven't noticed any significant speed change - But then I just installed it the morning so I can't really say. Not to mention I have nothing installed to measure page load speed on each page I go to, and not to mention there are sites with simple, fast loading pages while many (especially these days) are graphics heavy.
Oh - And your ISP connection. I have 50 down and 5 up so I have a "decent" connection (Time Warner) going out to my ISP, at least.
A good read:
http://arstechnica.com/information-...h-firefox-29-more-customizable-more-chrome-y/