Ublock does not install on phone with android 5.1

I have a smartphone (Fairphone 2) with amdroid 5.1 and firefox beta. In firefox I can download the u-block add-on but it does not install.
On my desktop (Ubuntu 16.04 with firefox) it works fine.
Do I have to uninstall the app Adblock Browser first?

A post was merged into an existing topic: [Support] uBlock Origin