Unity locks up while firefox bookmarks menu is opening

A common usecase for firefox in Unity is holding down the mouse button and dragging across the firefox top menu items - File, Edit, View, History, Bookmarks, Tools, Help. If you accidentally touch Bookmarks while dragging across the other menu items, the whole Unity interface effectively locks itself up until the bookmarks menu finally appears. With just 2000 bookmarks at top-level, clicking the Bookmarks menu button takes 5 seconds before the menu appears on recent, very fast computers with 8-core CPUs. This is a serious usability issue.

As the number of top-level bookmarks (bookmarks that are not stored in sub-folders) grows, the delay after clicking the Bookmarks menu button until the bookmarks popup menu appears gets longer. There is no disk I/O associated with the delay. It appears to be caused by purely CPU-bound activity.

It seems the delay is perhaps O(n^2) or worse in the number of bookmark items contained at the top-level menu.
This implies a poor choice or incorrect implementation of algorithms in sorting and / or displaying the bookmarks. With an efficient algorithm (ref. any good computer science textbook) correctly implemented, it should be possible to handle a vast number of top-level bookmarks without noticeable delay, perhaps 1,000,000s, until disk I/O starts to become an issue for first-run caching time.