Firefox can't start a program (Forte Agent) to open a downloaded file. Agent starts but asks for my registration key (I have one) and then goes into setup.
Sometimes I download nzb files, which are Usenet index codes. You open nzb files with a Usenet newsreader program, it understands the nzb data, and downloads the actual message. The newsreader program I use is called 'Forte Agent'. I have a legal copy of Agent version 7 (not the latest v8) installed on my Windows 10 PC, with my 'registration key' saved, and the program options set up as I like them. Agent works fine when I run it from the desktop.
When I was using Chrome (last week), I could download and "open" nzb files from Chrome, which automatically launched my Agent newsreader program. Then I could use Agent to fetch the message, and open it.
When I try to do this with the Firefox browser, I can get Firefox to open Agent, but it is as if I was just now installing Agent for the first time. Agent asked for my registration key, and accepted it, then it asked for me to start the Agent program initial set-up.
I tried configuring Firefox to launch the program from c:\program files (x86)\agent\agent.exe but this made no difference.
I am now saving all nzb files to my download directory. I can open the saved nzb files with agent, and they work fine.