Speed Up Firefox with a Click of Your Mouse
Vacuum Places Improved adds a small icon to the Firefox status bar that, when clicked automatically “vacuums” your Places database—the one that slows down Firefox when it gets fragmented. You can also automate the task by ticking the Automatically clean places checkbox in the Vacuum Places Improved extension Preferences and entering your desired number of Firefox startups since last cleaning.
It still doesn’t solve one of the biggest items on our Firefox wishlist (i.e., better memory management), but at least its an easy way to execute one of the workarounds.
Vacuum Places Improved is an experimental extension (so install with caution), works wherever Firefox does.

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[...] Vacuum your Firefox, Speed up Internet Explorer 7 – No, it isn’t the title of a Frank Zappa album, it is an alternate way of “defragging” Firefox for better memory management. I’ve also tossed in a link that offers general speedup advice for users of IE 7. Wouldn’t want to be accused of playing favorites. [...]