Firefox for Android is on a four week rapid release cycle. This means that usually there is a new release going out every four weeks.
Within this timeframe, localizers will usually have approximately four weeks to localize and test their strings for a given release: strings land continuously on Nightly during the first four weeks, and then these strings ride the train to the Beta version. The deadline in Pontoon is an indicator of when the next localization cut-off date is for the upcoming release - in other words, the last day to get localization updates into that release.
Note that all files contained in the project should be localized in order for Firefox for Android to be complete.
Localizers should go to the Firefox for Android page on Pontoon, and click "Request New Language". They will be notified by email once the locale is set up and the project is ready to be localized.
When the locale has been set up, localizers can choose it from the list of locales on Pontoon, and start localizing the product from there.
Once localization starts, the locale should become available in Nightly builds within the next few days, which will enable localizers to check and test their work (see this section for more information about testing).
When the locale reaches 70% completion (and given that active testing has taken place), it can be added to the release version.