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

@ngokevin [says here](https://github.com/mozilla/fireplace/pull/833#discussion_r21651432) the `.act-tray-mobile` -> `.act-tray-mobile .icon` change is making things harder to click. let's figure that out.