The PirateBrowser is very similar in many respects to the Tor Browser Bundle (which was recently exploited). As is the case with the Tor Browser Bundle, the open-source Firefox browser is at the core, though it’s not the same version or release branch.
The Tor Browser Bundle is based on the Firefox Extended Support Release (ESR) branch, while the PirateBrowser is based on the Firefox Portable Edition, currently shipping with the recently released Firefox 23 browser. Firefox ESR differs from the mainline Firefox release branch in terms of the release cadence for major milestone updates. Firefox ESR aims to deliver a longer term of stability as opposed to the rapid rate of release from the mainline.