One less dialog to deal with on new machines, please.
We could have a threshold for some data types like History that if you don't have more than N pages, that data type says it doesn't have anything (for startup migration only?). I guess if every data type we migrate from that browser is below some threshold then we could skip the migration screen but some data types like passwords the threshold would probably be >0. If we assume that no history means no relevant data then we will miss people who just cleared their history before moving to Fx or always use Private Browsing but have bookmarks (for example). Note that bug 670989 would make this dialog required anyways (though that's just an experiment for now). See https://wiki.mozilla.org/QA/Firefox_migrators#Supported_data_types for the data types migrated.
Start reading data for each data type before the dialog is shown would slowdown the startup quite a bit. If Firefox takes a lot of time to start we are not making a great first run experience, I'd say it would be worse than showing a dialog. Showing the browser ui and then the migration dialog may solve that, but it involves heavy refactoring of migration and I'm not sure we have the resources to spend there. Finally, we may have some hidden dependency on the order things run, for bookmarks import (preserve correct order of bookmarks) at least. But maybe that problem has already been solved when we refactored the component, I honestly don't recall and should be verified.
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