Actually, there are addons for firefox for that - tab kit (https://addons.mozilla.org/en-US/firefox/addon/5447/ https://addons.mozilla.org/en-US/firefox/addon/

Actually, there are addons for firefox for that - tab kit (https://addons.mozilla.org/en-US/firefox/addon/5447/ https://addons.mozilla.org/en-US/firefox/addon/5447/) and tree style tabs (https://addons.mozilla.org/en-US/firefox/addon/5890/ https://addons.mozilla.org/en-US/firefox/addon/5890/) Personally I use tab kit. However, for some reason I find it more convenient to have tabs on right.