Firefox pops up a dialog noting that it is not my default browser. The options are to make it my default, close the box, or click "Not Now". Note the absence of "No, I do not want this to be my default browser... So I've tried all combinations of answering that dialog, including checking and unchecking the "ask every time" box, and it has no effect.

Of course, I went into my options, and made sure the box to check for FF being default browser was not checked - it isn't. I even tried making FF my default browser, making sure it stopped asking, and then changed back to my other browser - Firefox resumed asking if it should be default.