fix `fitNavbar` error (bug 1104359)

https://bugzilla.mozilla.org/show_bug.cgi?id=1104359

was accidentally changed in https://github.com/mozilla/fireplace/commit/5f11a2e3#diff-702af7d747f885c2f09c915d09c9803eR181

r? @ngokevin 
@ngokevin do I need to add `fitNavbarDesktop();` below?