The **Tor Browser**, which is the most common way people access the Tor network, is built on Mozilla Firefox’s open-source code.

Uses a modified version of Firefox’s Gecko engine and is hardened with additional privacy and security features.

The Tor Project team customizes this Firefox base to integrate it with the Tor network, adding things like the Tor protocol, proxy settings, and anti-tracking measures that vanilla Firefox doesn’t have.

So, while the Tor Browser is derived from Firefox, it's not even close to being the same thing.