Starting with Tor Browser 16.0a1, the Tor Browser Alpha release channel will be based on Firefox Rapid Release rather than Firefox Extended Support Release. If you are an at-risk user, concerned about your privacy, or just need a web-browser that works reliably, you should not use Tor Browser Alpha and instead stick with Tor Browser Stable.
Tor Browser 15.0 is now available, featuring an update to Firefox ESR 140, improvements to tab management and the address bar on desktop, and screen lock for Android.