If you really did mean iOS on an iPhone, iPad, iPod Touch, then I only use Safari on my iPhone. The one thing that seems to work better for me on Safari is some videos load and run with less hassle, than Firefox. But I switched to Safari because of my Quicksilver addiction. I've found no problems with Firefox's performance or text rendering on the Mac, perhaps because I have a new (and, therefore, relatively powerful) system. I do use Safari as well (I actually have both up and running on my Macbook Pro). Mac zealots seem to prefer the Mac's native Safari over Firefox, citing speed and the way Safari handles text as primary reasons. This does slow down Firefox for me, but that is my choice.
I use Firefox mostly, but mainly because of the extensive number of plug-ins/extensions available that I want to use.
Different releases are sometimes faster than the other, and then the other gets faster.
If you do not install any plug-ins/extensions for each browser, they are both reasonable efficient. If this is for OS X running on Yosemite, then security wise, as long as you are applying all the security updates for OS X/Safari and/or installing the latest Firefox versions, then both products are kept safe and secure to the best of each development teams abilities. If this is really an iPhone, iPad or iPod Touch question, you should be posting in the appropriate forum for that product But you are posting in the OS X Yosemite 10.10 forum? You say iOS, which is the OS name running on the iPhone, iPad, and iPod Touch.