problem in using subprocess module

[akbarza]

I think the code is used for running Firefox from inside Python. is it true?
why did not the code run successfully?

Yes it will try to run the FireFox binary,but i think you misunderstand how this work.
As the Firefox(Browser) is a long running process and may give no output to stdout at all.
The eval make no sense here,can only use Python eval if output make any sense for this and are secure for task like this.

To give example of that make some sense,so eg when run ping on command line,
it give output about request messages to a target Ip.

import subprocess

response = subprocess.run(['ping', '-n' ,'4', 'python-forum.io'], capture_output=True, encoding='utf-8')
print(response.stdout)

Output:
Pinging python-forum.io [2606:4700:3035::6815:1b29] with 32 bytes of data:
Reply from 2606:4700:3035::6815:1b29: time=37ms 
Reply from 2606:4700:3035::6815:1b29: time=26ms 
Reply from 2606:4700:3035::6815:1b29: time=37ms 
Reply from 2606:4700:3035::6815:1b29: time=42ms 

Ping statistics for 2606:4700:3035::6815:1b29:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 26ms, Maximum = 42ms, Average = 35ms

So now in subprocess running the ping command and capture is output.

Also subprocess.getoutput() is not common used at all.

Using the subprocess Module

The recommended approach to invoking subprocesses is to use the run() function for all use cases it can handle.
For more advanced use cases, the underlying Popen interface can be used directly.

hi snippet
I am not familiar with the subprocess library and your code:

response = subprocess.run(['ping', '-n' ,'4', 'python-forum.io'], capture_output=True, encoding='utf-8')

how(where) can I gain more info about the above code?
do you have any opinion about my second question?
thanks a lot.