Well, what Que said is accurate. Mozilla is extremely privacy focused and has a philosophy whereby they promote things like open web standards as well as fight against government surveillance (insofar as they can).
Besides that, Firefox has caught up with Chrome regarding performance. They fixed that issue a long time ago. They also still have the largest number of extensions. They also do the same tab, bookmark, and history synchronization that Chrome does but Firefox goes a bit further and supports extension synchronization as well.
Where they are behind, though, is in overall security and process separation so that one tab with one JavaScript that is eating all of your CPU or RAM doesn't freeze the whole browser. They are working on both.
Chrome is a good browser, other than the RAM issue. I just like Mozilla's philosophy as well as its legacy.