Well, I used the 223 on called in foxes for a long time off, and on. My problem was trying to find a load that'd drop a coyote or bobcat, but still not blow a fox up too bad. Trust me, that's nearly impossible, and will run you crazy trying, but a heavier bullet at reduced velocities can give barely acceptable results.
If you can legally use FMJ bullets then that's exactly what you need to do for foxes. I'd "never" use them on coyotes, or bobcats, but foxes flop over pretty easy when hit with just about anything through the vitals. Even with FMJ bullets though, you'll need to keep the velocity fairly low. A foxes hide is like paper, and just can't contain the pressure from a high velocity bullet even with zero expansion. Run the FMJ's at around 2500-2700 fps, and you'll get an exit hole that's not too bad.
If you hit a fox with any polymer tipped bullet at nearly any velocity, you're gonna blow a huge hole in him at best.