How to activate multi-line command line in Firefox DevTools?

Firefox Developer version(v70) has multiline console which can be activated with ctrl+b (on linux at least, I havent found the shortcut in the offical docs). So this will soon show up in mainline FF i guess.

You can try the scratchpad. Go to Settings >> Available Toolbox Buttons and check the box for scratchpad. Refer the screenshot:

Once done, you'll have a notepad-like icon in your developer console navigation bar on the right side.... Click on it and it will open a scratch pad on which you can write code and execute...

Good Luck.

The command line the Firefox DevTools has a "smart multi-line" feature. That means it detects when a command is not finished and automatically inserts a line break, e.g. when you write document. and hit Enter. As you mentioned, you can also insert a line break manually by pressing Shift+Enter.

The DevTools also offer a completely separate command editor panel called Scratchpad, which you can enable via the settings (under the Default Developer Tools* section).

There is currently no multi-line editor as a side panel like Firebug's Command Editor, but it is already requested.

This is also described in the migration guide for Firebug users.