mozilla is roling out a fix
https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
that said they are using the 'studies' infrastructure to do so, and
they seem to be disabled in the debian build.
- Bug#928417: [firefox] All extensions are disabl... Antonio Russo
- Bug#928417: studies disabled in debian Niv Sardi
- Bug#928417: studies disabled in debian Tobias Frost
- Bug#928417: Temporary fix: set xpinstall.s... Felicia P
- Bug#928417: hotfix-update-xpi-intermedi...... Holger Schröder
- Bug#928417: Bug#928415,#928417: [firefox] ... Andres Salomon
- Bug#928417: studies disabled in debian Pedro Silva
- Bug#928417: marked as done ([firefox] All ... Debian Bug Tracking System