Ways to see slowness with http://del.icio.us/jesser or the testcase: * Just load the page, and hover over toolbar buttons to see that Firefox is unresponsive for a second or two during the load. * Make sure Firefox is set to not show the tab bar when there's only one tab. Load the page in the only tab, then middle-click the Home button. There is a delay of about a second before the new tab appears, and a similar delay if you close the tab. * Resize the window while the page is visible. bz says "most of the time is in reflow... 105586 nsBlockReflowState::AddFloat" (with the original page, not the testcase).
This is less painful now that del.icio.us lets me set a cookie to restrict which tags appear in the sidebar (the "use minimum" option at the bottom of tag lists).
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9a2pre) Gecko/20070118 Minefield/3.0a2pre WFM with the "fairly simple testcase", but http://del.icio.us/jesser is still bad :(
WFM on delicious due to delicious redesign.