User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:44.0) Gecko/20100101 Firefox/44.0 Build ID: 20160123151951 Steps to reproduce: I have selected `Don't load tabs until selected` in Tools > Options > General. I have YouTube open and I open a YouTube video in a new tab (using clicking on its title using middle mouse button). Actual results: The video tab opens and loads even without being selected. Expected results: The video tab opens and doesn't load until being selected. Firefox already has the possibility of loading tabs when they are selected and not in before - When Firefox starts and restores a session, this is what happens to the tabs. Please, also make it work this way when `Don't load tabs until selected` is checked and you manually open a new tab.

I don't insist on it being enabled by default and I don't insist on it even have its own place in the Preferences dialog window. It can just be a boolean key:value pair in the about:config. Something like: Also work on user opened tabs: true. I am well aware that there are some addons to do this, however the point is to not use any third-party/extra tools since Firefox already has the feature, just not working in this case.

On New Tab page only? Or for all pages? I think it will not be integrated, this is not a common feature, and we have add-ons. Try like the https://addons.mozilla.org/firefox/addon/load-tabs-progressively-fixed/.

By all pages do you mean also for link/page opened in new window? Yes it can be for all pages. The exact functionality you get when you restore Firefox's session - the way you get the selected tab loaded and the rest don't load until you select them.