WebService: restrict form's submit
FindAgainScroller: simplify scrollable node test
FindAgainScroller: improve visibility checks
UI: get rid of cache of DOM element

* It causes the memory leaks even if the default XUL elements
* Firefox renews all toolbars when the cusutomization panel opens. Then, our
  cache of the component of the URLbar died.
Misc: make function
minor fixup

* update comments
* descriptively rename variables: 'ex' means exception