You can submit patches by using Phabricator. Walk through its documentation in how to set it up, and uploading patches for review. Don’t worry about which person to select for reviewing your code. It will be done automatically.
Please also make sure to follow the commit creation guidelines.
Once you have contributed a couple of patches, we are happy to sponsor you in becoming a Mozilla committer. When you have been granted commit access level 1 you will have permission to use the Firefox CI to trigger your own “try runs” to test your changes.
This is a good try syntax to use when testing Marionette changes:
-b do -p linux,linux64,macosx64,win64,android-api-16 -u marionette,marionette-headless,xpcshell,web-platform-tests,firefox-ui-functional -t none