Error while installing Firefox Developer Edition on 18.04.1 LTS

Ubuntu's official repository and Ubuntu Desktop's repository contains umake versions 16.11.1ubuntu1 and 18.05~bionic1 respectively. This seems to be a bug under that versions of umake. I installed umake from the repository of Galileo Sartor and all seem to work fine under that umake. This repository contains umake version 18.09+201902042247~ubuntu18.04.1 at the time of installation. That umake installed Firefox Developer Edition version 65.0b5.

• Uninstall previously installed Ubuntu make and Ubuntu Desktop's repository:

  sudo apt autoremove ubuntu-make
  sudo add-apt-repository --remove ppa:ubuntu-desktop/ubuntu-make
  

• To install Ubuntu make from above said repository and then firefox-dev:

  sudo add-apt-repository ppa:lyzardking/ubuntu-make
  sudo apt install ubuntu-make
  umake web firefox-dev
  

This repository contains daily build versions of umake and is also mentioned in umake - Ubuntu Wiki, so might be considered as a trusted ppa.

Simply specify the language along with the umake command

umake web firefox-dev --lang en-US

you are welcome to replace the --lang en-US with the language of your choice.

Solution credits from Ubuntu umake issue tracker git