That's the first release of Enigmail/p≡p software since the collaboration was announced over 2 years ago. Work on Enigmail/p≡p was considerably intensified after August of last year by engaging further people working on and testing Engimail/p≡p.
When is p≡p activated?
p≡p gets automatically activated in Enigmail if there's no prior Enigmail configuration in place, i.e. for all novice users. New Enigmail users don't have to care about the creation of key pairs for their email accounts anymore. Public keys are automatically distributed to the users (attached). The subject line and other non-mandatory meta-data get automatically encrypted alongside the message body and attachments (with PGP/MIME and message encapsulation) whenever emails can be encrypted (i.e., principle of Privacy by Default).
How can p≡p be force-enabled if not active?
In case p≡p shouldn't be activated in Enigmail (e.g., because you already had an Enigmail configuration), you can force-enable p≡p by entering the Enigmail Preferences, clicking on the Privacy tab and choose to force-use p≡p.
Are there cases where p≡p won't work in Enigmail?
Yes, p≡p needs either GnuPG v2.0.30 or 2.1.17 and newer to work. This especially affects Ubuntu users with Ubuntu 16.04 or older, which are equipped with incompatible versions of GnuPG. This will definitely be fixed for Ubuntu 18.04 users (cf. this Enigmail forum post for more information).
Also Debian users will experience issues if they install Enigmail as Debian package, because the silent-install of p≡p is deactivated. A work-around for now is to directly install Enigmail from the add-ons manager of Thunderbird instead.
For macOS users there might be issues with conflicting GnuPG software already installed (cf. bug 766): generally, however, both macOS and Windows users will be served with suitable GnuPG versions provided by Enigmail. Enigmail will help you to install GnuPG if not found in any suitable version. After that, p≡p should be activated by default.
In all above cases, however, we're working on solutions and work-arounds.
What can I do if I run into issues using Enigmail/p≡p?
Which major improvements on Enigmail/p≡p can be expected next?
Despite integration work of Enigmail/p≡p into Mozilla Thunderbird which can now be started, we're also working on having p≡p key synchronization (KeySync) working with Enigmail. With KeySync you can easily transfer your secret key material across different devices for a specific email account running on any other device you use with that address (e.g., for an email address used in parallel in Android, iOS, Outlook or any other instance of Thunderbird). With that, you retain the possibility to read and send encrypted emails across all of your devices you use email with.