Here's the skinny: Firefox stores your history, bookmarks, and most else in what are called SQLite databases. If you know what SQLite means, this is probably not the blog for you. I for one have no idea why they're called that. Regardless, like any other databases, these SQLite ones get fragmented over time. This has real potential to screw up your page and search bars' loading times. Mozilla recommends trimming and defragmenting these files every so often. Vacuum Places Improved (we can only imagine how it must have sucked before) is a simple and automated means of doing just that.
It's experimental, but be bold.