Naming your extension and choosing a target browser

The gear icon above the preview opens Settings, which is where the details that end up in your manifest live.

Extension name

What users see in the Chrome Web Store and in their browser. Changeable at any point.

Version

Standard version numbering, for example 1.0.1. Worth incrementing when you publish an update, because the Chrome Web Store expects the number to go up.

Description

The short description of what your extension does. This feeds your store listing, so it is worth writing properly rather than leaving whatever was generated.

Target browser

Chrome by default.

There is also a Mozilla Firefox option, currently in beta and described as manifest only. That is worth taking literally: choosing Firefox adjusts manifest.json for Firefox, but the generated code may still use Chrome-specific APIs that need changing by hand.

So it is a head start on a Firefox version rather than a finished one. If you need Firefox support, expect to do some work after the export, and test it properly before you ship it.