I do not use debian but hopefully I can be of assistance.
I believe there is a way to convert rpm's to deb packages and there are instructions here.
However, I am not aware of any recent firefox updates, and am thinking that the newest version of firefox should be available using apt-get.
It is odd that the ubuntu version of firefox is slow. It should be optimized for ubuntu.
On a side note:
I generally find Firefox to be a slow browser in general. Since kde 3.5 I switched to konqueror which is an awsome browser/file manager. (it is very usable if you customize the interface) I also like opera since it is fast.