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I wrote a XUL test file to profile the impact of updating the accessible tree cache more frequently. The idea is to see if it is worth trying to optimize how we update the core tree accessible caches. I currently only regularly build Thunderbird trunk. So in order to run the test I needed to load the file in tbird. After poking around the GUI... no luck. I headed over to #maildev and #xul... mrfinkle and I chatted about how to solve the problem via the creation of an extension. He pointed me to a handy extension wizard, as well as information on using an extension in test mode. I'm noting this for a rainy day as I'm sure it will come in handy but I also heard from philor over on #maildev who had run into the same problem before and like me didn't want to create an extension just to test 1 file.
His solution? Set the start page in the preferences to be the XUL file. Nice.