Firefox Test Pilot program has been ended and two days back on January 22, 2019 the Test Pilot team has posted a goodbye message on the website, if you’ve installed Test Pilot extension, the extension will be uninstalled automatically when you visit their website and your current experiments will be migrated from Test Pilot to add-on versions available on AMO.
These Test Pilot Experiments are now available on Add-ons.Mozilla.org as extensions for download and you no longer need to install any add-on to prior to test or download these: Firefox Notes, Email Tabs, Side View and Color. Price Wise not yet available at the time of writing, we’ll update its link when Mozilla publishes the add-on in AMO [update: done].

UPDATE February 02. 2019: Price Wise AMO link has been added.
Which is the Test Pilot experiment you like the most and using now in Firefox browser?
Related articles:
Firefox Test Pilot Notes v3 Receives New Features
Firefox Notes Test Pilot add-on gains dark theme and a Keyboard Shortcut
Min Vid: Video Minimize feature coming to Firefox Test Pilot Program
Firefox Test Pilot’s Universal Search Experiment retires