I know Mozilla thinks it's a bug that IE shows ALT text even after the image has loaded; while users think it's a bug that Firefox doesn't work exactly like IE. This disagreement is irrelevant to my question:
How can I make my Firefox show the ALT tooltip when I hover the mouse over an image?
Obviously XKCD is a bit tame without this essential functionality.
I've tried the Firefox plugin Popup ALT Attribute but it doesn't work; after restarting the browser and hovering over an image with an ALT text, no tooltip appears. I reproduced this on several computers with several (Windows) OS versions.