Sorta. There were actually three Skywriters: Bespin prototype (Thunderhead-based), Bespin Reboot (SproutCore/jQuery-based), and ACE. ACE postdates the rename from Bespin to Skywriter. They originated as three completely separate codebases, although there's been a fair amount of cross-pollination.

ACE didn't begin as a Mozilla project, although it was finally chosen as the basis on which to build Skywriter the product (which will be used for the Firefox Add-ons Builder, in particular).