Noticed this site was causing firefox to hang for about a minute and peg my CPU, so I profiled it:

The Addthis script you're using has an onReady method that gets called 94 times on pageload and takes an average 12 seconds to run each time. During this time Firefox gets the dreaded beachball/umbrella.

I'm having similar issues on a site we're using sharethis on, but not quite that bad on page load - it does increase the constant load on the cpu by about 15% though.

Be careful when using these widgets...

(macbook pro; 2.4ghz; 4gb ram; ff 3.0)