Watch Out for Too Much of a Good Thing
All of the add-ons discussed here are free--but they can still cost you. Each uses a certain amount of system memory and could increase the time it takes your browser to start up. Add too many, and you'll notice a definite slowdown.
In informal tests, we compared the amount of memory used by Internet Explorer and Firefox with and without the Google Toolbar, FoxyTunes, SiteAdvisor, and Trailfire add-ons. 
Also, some add-ons don't play nicely together. It's almost impossible to test the whole universe of possible add-on combinations in advance, but using two incompatible ones could break your browser.
If your browser is acting up and you want to see whether your extensions are at fault, try starting either browser without add-ons: For IE 7, click Start, All Programs, Accessories, System Tools and then select Internet Explorer (No Addons). For Firefox 2, click Start, All Programs, Mozilla Firefox and then choose Mozilla Firefox (Safe Mode). Then you can selectively disable particular add-ons by using the add-ons manager in either browser.
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