stop navbar-linefitting event listeners from getting called again on every page load (bug 1109448; bug 1059002)

https://bugzilla.mozilla.org/show_bug.cgi?id=1109448
https://bugzilla.mozilla.org/show_bug.cgi?id=1059002

I could cache these in a listener when page loads (`z.page.one('loaded')`) and have some global variables. thoughts?