[fix bug 1106065] Add version check for getConfiguration('selectedSearchEngine') on firstrun

This PR is just tying up a couple of loose ends on /firstrun. I don't really consider any of this urgent and it can wait until after Mon 1 Dec.

* Adds a version check when calling `getConfiguration('selectedSearchEngine')` to suppress an error in the Browser Console on older Firefox versions (note this is not the regular Web Console). The error does not impact anything functionally.
* This PR also removes some now redundant `appMenu` CSS class names from /firstrun which are no longer needed (I didn't consider this to warrant creating a separate bug for).