First you have to find the extension ID. There are several ways to do it. You can navigate to chrome://extensions, turn on developer mode in the upper right corner, and ID will be shown in the box for each extension. Or, you can simply find the extension ID from the url on the Google Chrome Web Store. In case of BPIP Extension, the extension ID is: ecpfdlfjiabpdnlhmkmannofnmpdakkj

Now go to the folder where the extensions are saved on your system. If you are using Windows, it is: C:\Users\[Your_User_name]\AppData\Local\Google\Chrome\User Data\Default\Extensions\ and find the folder with your extension ID. There you will find the extension's source code.

Of course, this is for the Google Chrome browser, the same applies to the Mozilla browser, but I don't know the exact path.