What is First Run Release Date?
The first run release date field is applicable for mobile first run experiments. If you are running a first run experiment, selecting the "First Run Experiment" checkbox will allow you to select a release date. When entering the date, you should select the expected release date for the Firefox version that you are targeting (this version number is what you are selecting in the "Min Version" field at the top of the Audience page).
If you do not know the release date, never fear! The help text on the "First Run Release Date" links to https://whattrainisitnow.com/, where you can look up the dates for whatever version of Firefox that you are targeting.
Why should I care about this date?
First run experiments are bundled with the app itself, so it's important that we know when the specific app version will be released to the users. This will help us notify you when it is time to end enrollment/end the experiment.