Fix EZP-23099: In Firefox, the height of the tree is not limited

JIRA: https://jira.ez.no/browse/EZP-23099

# Description

In Firefox, the height of the tree was not limited. This happened because of a misuse of the `setStyle` method ([see documentation](http://yuilibrary.com/yui/docs/api/classes/Node.html#method_setStyle)) which is working in Chrome but not in Firefox.

# Tests

manual tests

Note: the unit tests did not spot that because currently we are only using PhantomJS (*same* engine as Chrome/Safari) to run the test, there's a [story for adding SlimmerJS](https://jira.ez.no/browse/EZP-23732) (same engine as Firefox).