Yes the new Firefox is slow but I don't think it's the be all end all of crap that others have made the update out to be. Maybe I'm just content with what I see as the lesser of two evils.
On my Windows desktop and my Galaxy Tab tablet the new updated Firefox works well, it just seems the android version has compatibility issues with the ASUS Transformer Pad TF300T.
My Transformer Prime has issues with Firefox as well. Hopefully the next update will fix things. Although, my tablet has been running a lot better since I disabled a lot of the Asus built in apps. If only I could completely remove them...without rooting it and whatever other arcane magic is involved.