The Web Developer menu is the main way to access the developer tools that are built into Firefox. On OS X and Linux, it's under the "Tools" menu:
On Windows 7, it's under the "Firefox" menu:
You'll see that the menu is split into three sections:
- the first section lists tools that are hosted in the Toolbox, which is a dedicated window for developer tools
- the second section lists integrated tools that are not hosted in the Toolbox, as well as any tools installed as add-ons (you'll see in the Windows screenshot above that I've installed the Firefox OS Simulator)
- the third section, "Get More Tools", is a link to more web development add-ons for Firefox