./mach build
Next, open <code>embedding/ios/GeckoEmbed/GeckoEmbed.xcodeproj</code> in XCode . Change the target to some variant of the iOS simulator (click on the GeckoEmbed icon just beside the play and stop buttons), then select Product -> Build For -> Running. You should then be able to do Product -> Run if the build was successful, and the simulator should open eventually showing mozilla.org.
The XCode project has an external build step to rebuild Gecko using <code>make binaries</code>, so once you've done a full Gecko build you can edit and rebuild from XCode.