Editting Firefox source

For our most recent lab in OSD600 we were supposed to do 2 things.  The first was make it so opening a new tab would open beside the current tab as opposed to at the end of the queue of tabs.  The second was to change the key to cycle through active tabs from ctrl to alt.

I managed to find the location of the xml file to change the functionality of how the tabbing operated after a bit of googling.  Now the real problem began when I tried to view my changes in action.  Apparentley just changing the files and re-opening your browser doesnt cut it.  I tried committing my changes locally (as thats what you need to do apparentley in mercurial) and then tried re-opening the browser, but when I did this, my source built firefox decided to die.  After this I could not even open my source built firefox.  I tried reverting to old commits (ones previously commited before I began altering the code) and still nothing.  I guess I will bring up what happened tomorrow in class and see if anyone has any suggestions.