What is JavaScript Executor in Selenium WebDriver?

What is the use of it and how can we use this in Selenium WebDriver?

Example will be appreciated

I looked at all the possible keys in all json files based on the answer in Python Selenium - What are possible keys in FireFox webdriver profile preferences, but I couldn't find a key to specify the client certificate to use in my SSL connection.

I have researched on this, but I couldn't find the exact answer. I found that we need to add the certificate to the FireFox profile based on the answer in How to import SSL certificates for Firefox with Selenium Certification [in Python]?, but I am kind of stuck here, I can't figure out how exactly this certificate needs to be added.

Please note that I am not talking about trusting a server's certificate. By default, when you initiate an SSL connection, a local certificate that is assigned to your workstation is used as the client certificate. Here I need to use a new certificate/private key pair for my SSL connection. I need to do this to test client authentication in SSL.

So, in summary, I am looking for some configuration that looks like this:
<pre>
profile.add_client_cert(path_to_cert)
profile.add_private_key(path_to_private_key)
I found a couple of files that might be what I need to tweak, but not sure how to add certificate and keys to these files, cert8.db and key3.db. I found these files in the FireFox profile directory.</pre>

I searched the source code of selenium but couldn't find the answer: https://github.com/SeleniumHQ/selenium/search?utf8=✓&q=cert