Hey, I'm seeing this too on Firefox Mac 3.0.11. I systematically added in my firefox add-ons (plugins). It appears to be caused by Delicious Bookmarks 2.1.041.
I was hoping that it was caused by a developer plugin so it wouldn't effect end users as frequently. But, on my machine it does *not* seem to occur with YSlow 2.0.0b3, Firebug 1.3.3, MeasureIt 0.3.8, Selenium IDE 1.0b2, Web Developer 1.1.6, or Google Toolbar for Firefox 3.1.20081127M.
I have a lot of Delicious bookmarks and I noticed that my browser speeds up when I disable the bookmarks. There might be some issue that shows up due to the browser slowdown caused by lots of Delicious bookmarks.
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