If you have used Chrome then you have seen that when you see that pressing CTRL + U show the page-source in New tab.
The firefox have same feature but it’s open the view-source in pop-up instead of new tab on right.
So in Firefox if you want to see the page-source in new tab then you can install Tabextension plugin in Firefox to get this done.
For installation you can got the plugin from here.
http://piro.sakura.ne.jp/xul/tabextensions/index.html.en#what