Epiphany is a default browser of the GNOME Desktop (and my favorite one). Traditionally it used Gecko HTML Rendering backend to display web pages. Recently WebKit renderer got a lot momentum and Epiphany got support for it. (There is a long lived desire in Epiphany community to replace Gecko with something that does only HTML rendering and nothing more.)
WebKit is not the default backend yet. It’s stable, but has much usability quirks. It’s not supported by popular distributions yet. But you may want to try it yourself now…
First you need to add ‘misery’ repository:
sudo echo 'deb http://ppa.launchpad.net/misery/ubuntu/ hardy main' > /etc/apt/sources.list.d/misery.list sudo apt-get update
Then update your epiphany to latest version if you have it installed already:
sudo apt-get update
Next install webkit based backend for Epiphany:
sudo apt-get install epiphany-webkit
Finally make it your default browser:
sudo update-alternatives --set epiphany-browser /usr/bin/epiphany-webkit sudo update-alternatives --set gnome-www-browser /usr/bin/epiphany-webkit
This instruction was written using Epiphany running WebKit backend. :-)


Is it possible to get the extensions working?
Not yet.
Of course it is possible to fix them programatically by obtaining the source and applying programming knowledge, but I think this is not exactly what you ment. :-)
Do you know how to install Webkit Epiphany on jaunty ? (noob here?)
epiphany-webkit is available in official universe repos for Intrepid. http://packages.ubuntu.com/intrepid/epiphany-webkit
I don’t know why there isn’t one for Jaunty.
There is the webkit-team PPA for Epiphany Webkit on Jaunty. I posted some instructions to get started – similar to these, but include adding the PPA key: http://mattaustin.me.uk/2009/06/epiphany-webkit-on-ubuntu-jaunty-9-04/