The Tor Browser bugs 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

The Tor Browser bugs 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. This is from the article. Although these features may be beneficial for Firefox, they may not be a part of their priorities. By Tor making their own fork and doing all the hard work (designing, coding and testing), they can just give Mozilla some patches. This only leaves a bit of their own testing, maybe some marge conflicts, so that the Firefox devs don't need to do the design and initial coding. This means that both guys win.