If you haven't already, install the Requestly Desktop app from the link above. In the app, find your proxy server information in the top bar. Note the host (usually 127.0.0.1) and the port number (usually 8080).
Click Save Certificate in the app and double click it to install. Open Keychain Access and set the certificate to trusted.
In your Network preferences, set the HTTP and HTTPS proxy to the values you got from the app.
Restart the browser, if required, and now all your network traffic would be passed through Requestly.
When you are done using Requestly or any network inconsistency appears, simply unset the system proxy.
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