Preferences

Settings menu > Preferences

Multi-instance

Allows you to open more than one window at a time:
Multi-instance > Always in multi-instance mode

List Tabs On The Left

General > Document List Panel > Show

In addition to the top-tabs for each document, you'll get a left-side panel list of them.
You can navigate with either.

Bug: Saving Files With No Extension

By default, files are being saved with no extension.

To fix it:
Settings > Preferences > Default Directory > uncheck "Use new style save dialog"
Sessions

A session is the set of files currently open.

Save current session: File menu > Save Session
Load previous session: File menu > Load Session