Something I failed to notice in the spec:

If the user is opening the import wizard, we want to take the most recent browser window (or open a browser window), and open a tab to about:preferences, and open the migration dialog in the center of that tab as an about:preferences SubDialog.

The sole exception to the above is in the startup migration case, where we will still open the dialog in a standalone XUL window.

There's prior art for how to open a SubDialog in a new or existing about:preferences in bug 1785282. The sole difference here is that we need to account for the case that a browser window doesn't currently exist and open one if we don't find one.

I'm responsible for missing this when looking at the spec, so I'll take this.

This does a few things to support this new behaviour:

1. Adds the ability for about:preferences category modules to indicate that
   they want to specially handle subcategories. In this particular case, it
   causes the "migrate" subcategory of the "general" category to open up the
   migration dialog.
2. Updates MigrationUtils to open about:preferences#general-migrate if the opener
   is a tabbrowser window.
3. Adds some utility functions to make writing tests easier.
4. Updates existing tests to expect the wizard to be opened in the about:preferences
   subdialog.
5. Makes pressing "Escape" close the migration wizard when it's loaded in the
   migration-dialog.html document.

https://hg.mozilla.org/mozilla-central/rev/d3d002fe950c
https://hg.mozilla.org/mozilla-central/rev/b9df29e90dfc