Firefox is currently not supported directly in Join.
You can still use basic Join functionality in Firefox by navigating to the Web Version of Join.
I’m working hard to get Join working on Firefox
It’s proving to be hard to send push messages to it, which is a basic functionality Join relies on to push pages, text etc.
You can follow the progress of me trying to implement it here.
I’m sorry for any inconvenience and lets hope I can get it to work soon!
Progress! I got push messages to go through to Firefox!
Sorry there’s not been any more development, but this is not forgotten. I have created a GitHub project that you can contribute to here if you want.