I recently upgraded to the new Ubuntu 18.04 LTS 'Bionoc Beaver'. By chance I have now discovered that it has disabled the Mozillateam PPA in "Software & updates" -> Other software: ppa.launchpad.net/mozillateam/ppa/ubuntu But why? Can I simply enabled it again, or will it do so by itself, when it considers it safe?
Thank you.
Ubuntu disables all PPAs and unofficial repositories during a release upgrade, to ensure it does not install incompatible packages.
If the respective PPA or repo provides packages for your new Ubuntu release version as well (i.e. bionic in your case), you can of course re-enable it after the release upgrade was successful and install/update your packages from it.
According to https://launchpad.net/~mozillateam/+archive/ubuntu/ppa, the specific PPA in question supports bionic, so you're good to go.
You might also want to have a look at Is there a tool to update my PPA sources.list entries after upgrading?
All third party sources are automatically disabled during the upgrade process. This is to avoid conflicts while upgrading. Be reminded that PPAs are not supported officially in any way and for a smooth upgrade experience it makes sense to disabled them.
Also, quite often after an upgrade, those sources are not valid or not even needed anymore and therefore need to be revised by the user.
In this particular case you can simply activate the PPA again, because it offers a version for 18.04 as well. It will never be automatically activated.
You might want to read What are PPAs and how do I use them? for a bit more in-depth view on the topic.
