I see this question was addressed before, but I don't understand the answer given.

I take it there is no "Save" button anywhere, nor any menu with a "Save" feature?

Playing on a Windows desktop computer, I was in the endgame in a single player game on a large map when my browser (Firefox) had a "hiccup" (or maybe I hit the wrong key?) and the tab I was running the game in disappeared.

Total newbie here; I have no clue as to how to restore that game. Well, I suppose it's too late now as I started a new game, but for the future it would be nice to know how to get back to a game if the browser tab closes unintentionally or if I have to shut down the 'puter.

go in single player, it'll ask you "would you like to resume your game ?" and push "yes" button.

But it works only for your precedent game ; you can't save many games at time and if you start another game, your precedent will be deleted. There's no way to save a single-player game. You can only save the template.

Note that games are saved in local storage/cache of the website, so if Firefox automatically clears those you won't ever "save" a single player game.

And due to that save system, it's also only saved locally, so won't transfer to another device or browser.

Also, check this link if you think you've lost your progress: https://www.warzone.com/singleplayer/savedgames

This is great, I didn't know that !