Regarding the new theme, I switched to Pale Moon partly because I was sick of trying to work around the latest pointless UI changes in FF with about:config tweaks and add-ons. The last straw for me was when the changed the download manager to the hybrid library view one they're using these days - click on download button, no, I can't see my recent download list yet, click again on 'show all downloads' or whatever, ah, finally. Improvement not found.
You can get rid of the new skin...they tell you how.
As for how FF29 runs ? Still has huge memory leaks. 1 tab, it showed 890MB
"Uses lots of RAM" or "uses more RAM than I think it should" is not the definition of a memory leak.
http://en.wikipedia.org/wiki/Memory_leak
in use, and things got really, really slow.
Took it 3 mins to exit completely, and this is on a 4GHz machine...
I'm not sure I've ever seen this happen in Firefox (and I've encountered many problems with it), except where a customer has a tonne of add-ons (usually inadvertently) installed (even then, not that complete combination of symptoms).
One thing I'd try is emptying the disk cache, sometimes older versions of FF would lose track of the contents of it, it balloons to a couple of GB and FF's performance goes right down the toilet. I've seen the disk cache problem happen in other browsers too, and every time I'm attempting to diagnose a browser performance issue, I check this.
Another test option would be to get FF to create a new profile (you don't have to delete the old one), and see whether your problem exists there as well. If you still have problems, I suggest starting a new thread with the symptoms and if I see it I'll try to help.