I'm using macOS Sierra (10.12) and Safari 10.0.
I usually just press ⌘N to open a new window in Safari. This shortcut seems to have changed recently (perhaps when I upgraded to Sierra). Now, ⌘N will only open a new window in non-full screen mode. In full screen mode it will open a new tab instead.
I checked under File in the menu and it displays the shortcut ⌥⌘N for New Window when I'm in full screen mode. This command works in full screen mode but if I'm not in full screen mode ⌥⌘N will open a new tab :S
I tried setting ⌘N as a keyboard shortcut for New Window in Safari (in System Preferences). Now it also says "New Window ⌘N" under File when I'm in full screen mode but the shortcut still opens a new tab.
Is this a bug or is it supposed to be a new feature? It doesn't seem to work very well. I would like to have a keyboard shortcut that opens a new window regardless of whether I'm in full screen mode or not.
Note: ⌘T seems to work the same way as before.
There is a setting in System Preferences > Dock called "Prefer tabs when opening documents". Change this to "Manually", it's probably set to "Full Screen Only" mode right now.
Not sure why this setting is under the Dock preferences but this will fix your issue. Downside is I think all applications behave that way now, like Finder and Preview. I guess you can't pick and choose specifically how you want an app to behave.
I turned it off because I am trained to use Cmd+N for new windows, and Cmd+T for tabs. It makes little sense why Cmd+N and Cmd+T would both open tabs regardless of settings. Why have the command execute from 2 different keyboard shortcuts, especially by wasting a classic shortcut that many have engrained in their heads.