Basically what I need is a way to automatize the result of the following operations:
open a new tab;
open the Network tab in the develo
By now there's a browser plugin to do that: https://github.com/devtools-html/har-export-trigger
It uses the WebExtensions DevTools API and I got it to work with both Firefox and Chrome.
See my code for Chrome here: https://github.com/theri/web-measurement-tools/blob/master/load/load_url_using_chrome.py#L175
Automatically installing the plugin in Chrome is a bit more complicated than in Firefox, but feasible - I extracted the plugin archive locally and then link to it in chrome_prefs.json (see same repository).