Steps to reproduce:

• open Firefox 2.0.0.20 (the latest stable version of Firefox 2)
• create a page tree with 15 or more levels of nesting
• on the deeply nested page click 'View in Hierarchy'
• the page tree takes a very long time to expand (~10 seconds for 15 levels, going up exponentially as you add more levels).

The page tree gets progressively slower in Firefox 2. This seems to be because it is expensive to call element.offsetHeight (or .clientHeight).

At the moment, it isn't something we can fix without rewriting much of the tree component. We recommend people experiencing this problem upgrade to Firefox 3 or later.