The Tor project forks Firefox
The Tor Browser bug [fixes] on the other hand are more directly usable by Firefox in its own Private Browsing Mode, which makes them more likely to merge quicker, and be maintained long-term. Also, because we are releasing our own Firefox-based browser, we will also have more control over experimenting with them and deploying these fixes to our users rapidly, as opposed to waiting for the next major Firefox release."
