I know that's the answer for running multiple sessions at once, but even if you're a tmux expert I don't see how people live with only one window. I constantly have multiple iterm windows open and frequently open new windows for one-off things.
If you're a tmux expert, it'd work something like this:
1) You don't have the default config/bindings.
2) You use some plugins.
3) You use multiple _panes.
4) You use multiple _windows_ (Do not confuse these 2).
5) You save sessions which persist between tmux sessions/reboots.
6) You run your terminal full screen or at least maximized.
Example:
I make a pane with ctrl+a (I don't use ctrl+b as tmux hotkey) plus - or plus |.
I make a window with ctrl+a + c.
I can switch between panes with ctrl+a + hjkl (or pointer).
I can switch between windows with ctrl+a + 1234567890 (or pointer).
So what matters is that running multiple terminal windows side by side is akin to using Tmux panes, and using multiple terminal tabs is akin to using Tmux windows.
It requires some practice to get used to it because basically Tmux becomes more and more your window manager...
(You can even run Firefox in a terminal btw, with Browsh.)