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@Flávio Amieiro
MeasureIt is an unnecessary extension to have if you install the Web Developer Toolbar. Web Developer Toolbar includes a ruler as one of its features. Under the "Miscellaneous" category for Web Developer click the option "Display Ruler" to use a ruler identical to the MeasureIt one.
That will allow you reduce the number of extensions needed by at least one.
+1 for LORI ("life-of-request-info"). It's a very convenient alternative for rough measurements of the load time of a particular web page -- the kind of thing that you might otherwise use an external stopwatch for.
New Tab Homepage. Combined with a "speed dial"-type homepage (a personal, fast-loading page of links that you use frequently), helps you get where you're going faster when you open a new browser tab.
LastTab. Changes the behavior of Ctrl+Tab to let you navigate back and forth between your most-recently-used tabs with repeated presses of Ctrl+Tab, the same way that Alt+Tab works in Windows. Also provides a nice view of all open tabs while Ctrl is still being held down for easy navigation. (The resultant behavior is very similar to the Ctrl+Tab behavior in recent releases of Visual Studio.)
I find Hackbar to be quite useful. Very useful if you want to edit the querystring part of the url, to test for vulnerabilities, or just general other types of testing where you might end up with complicated query string values.