I wanted to make simple app using KaiOS. I am following portal https://developer.kaiostech.com/first-app/softkeys
I made KaiOS environment setup and installed simulator as specified in portal https://developer.kaiostech.com/
KaiOS simulator(B2g) is started and able remote run time is also became successful in WebIDE but play button is disabled to run/install project on KaiOS simulator.
I am using Ubuntu 18.04.1 LTS and firefox quantum 61.0.1(64bit)
You have to install Firefox 43. Just Google it and you can find the mozilla archives. It took me a long time figuring this one out but finally I can code. Have fun!