wrapFirefox: fix default-browser update

Set MOZ_APP_LAUNCHER for firefox as per [1] (see [2] for detailed discussion).
Firefox will recognise itself across verions, skipping the 'not-the-default-browser' prompt.
Firefox will also write sane paths to the generated desktop file, should someone ever set it as default through the 'not-the-default-browser' prompt.
Also removed the unnecessary libtrick cruft.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=611953
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=593948