Like to use the “View Source” option in Firefox a lot? When you do this neat little option, it always opens in a new window. That can be a little annoying. What if you could get the source to pop up in the same tab? That is the action that the View Source Code in the Same Tab add-on does.
Here is some more information about that add-on from the developers:
Switch between a web page and its source code easily without opening another window, just in the same tab you are. Press: Control + Shift + s and enjoy! Then press the same shortcut to gte back the webpage.
You can also customize the shortcut. If this interests you, check it out at the Firefox Add-ons website.