You can use a Shift + left-click to open a link in a new window.

Command + left-click will open the link in a tab and you can't change that.

Thanks. It helped. I have tried Alt and Control, but didn't expect Shift to change behavior.

Then again, considering multi-window browsing use-case,

1. It is undiscoverable. Hardly ever Shift alone qualitatively changes behavior of a gesture. Thus it makes the gesture less discoverable. I even had to post question here after an amount of googling.

2. It is inconsistent. Other browsers make possible multi-window browsing with one or two simple settings. In Safari those are "never use tabs instead of windows" and "⌘click opens window". In Opera — "use windows insead of tabs". (Chrome doesn't support windowed browsing at all though).

Both browsers redefine ⌘click to open in new window. Safari uses shift+click to add to reading list instead. So Firefox behavior is inconsistent with alternatives and it makes switching harder.

Related question: https://support.mozilla.org/en-US/questions/758539

3. Requires changing habits. Suppose you want to switch from tabbed browsing to windowed browsing (my case). Firefox's behavior requires to change to gesture you use to open links. It may be quite hard to fight habits.

On the other hand, an option turning meaning of all gestures upside down (tab instead of window/window instead of tab) would make the switch transparent and intuitive. Intuitive I mean in the sense that with the options you don't redefine action bindings (action for middle-click, action for target="_blank", action for ⌘click) but you just choose your prefered mode of browsing: windows or tabs. Tab or Windows.

I'd like to hear any comments regarding what the issue is worth.

Additionally, in spite of the flags, mentioned in topic, no modifier key seems to make search results opened with [Context Menu → Search Google for ...] open in new window (they are opened in a tab).

Do you know a way to change this behavior?