I've a laptop on which Windows 10 Home Single Language 64-bit Operating System is installed.
I've installed the web browser 'Firefox Quantum 63.0 (64-bit)' on this laptop.
Then, I installed the web browser 'Firefox Developer Edition 64.0b4 (64-bit)' on this laptop.
I downloaded the 'Firefox Developer Edition 64.0b4 (64-bit)' installer file from the web browser 'Firefox Quantum 63.0 (64-bit)' and started the installation process by clicking on the downloaded installer file.
During the installation process only a dialog box appear having the below message :
Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window.
Then I closed the web browser 'Firefox Quantum 63.0 (64-bit)' and then the installation of web browser 'Firefox Developer Edition 64.0b4 (64-bit)' got done.
Also, when I open one of the both browsers and then try to another browser the dialog box with the same message appear, I've to close the opened browser and then the other browser can be opened.
Few days ago, this was not the problem on my laptop. I was able to use both the Firefox browsers at the same time on my laptop. But, I don't know why and how this problem suddenly started.
For your reference below is the
Locations of the .exe files at which both of these browsers are installed are as below :
'Firefox Quantum 63.0 (64-bit)' : "C:\Program Files\Mozilla Firefox\firefox.exe"
'Firefox Developer Edition 64.0b4 (64-bit)' : "C:\Program Files\Firefox Developer Edition\firefox.exe"