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. You can use the following try preset to run the most relevant tests:
% ./mach try --preset geckodriver
This preset will schedule geckodriver-related tests on various platforms. You can reduce the number of tasks by filtering on platforms (e.g. linux) or build type (e.g. opt):
% ./mach try --preset geckodriver -xq "'linux 'opt"