My clutch seems to have gone out tonight. The clutch is very easy to push to the floor...and it pops back....there's no pressure at all. The clutch fluid is leaking from the hose. Could this be the master cylinder? or slave cylinder?

I am not a mazda guy but it sounds like the pressure plate is bad. Have you check the clutch linkage to be sure something did not slip or break. If so I would be looking at the clutch pressure plate.

Well, it would shift easily with engine off, because the transmission input shaft is not turning. Engine running and clutch engaging good, the input shaft would again be stationary and allow smooth shifting.
What you have to determine is if the clutch disc or pressure plate is bad (a new clutch is needed), or if the pedal hydraulic system has a malfunction. When you push the pedal down, a rod from the top of pedal pushes into the clutch master cylinder bolted to the firewall under the hood. This cylinder has a reservoir filled with brake fluid. When the rod forces fluid from the clutch master down a steel line and into the clutch slave cylinder down on the transmission bell housing, the hydraulic pressure forces a plunger from the slave cylinder to extend and push the clutch fork lever a small amount, enough to force the clutch release bearing (aka throw-out bearing) against the pressure plate. This relieves pressure on the clutch disc and input shaft becomes stationary for easy shifting.

The clutch master cylinder must have gone out, the seals in it have failed. The clutch master cylinder (next to the brake master cylinder) gives you hydraulic pressure so the clutch pedal can operate the clutch.
You can buy a new clutch master, a rebuilt unit, or yours can be rebuilt with new seals. Save time and money and buy a rebuilt or remanufactured unit. Once installed, you bleed the system down at the clutch slave cylinder, mounted on the transaxle.

Probably bad clutch master cylinder and/or slave cylinder. I always replace both.

either the slave or master cylinder (or both ) may need an overhaul , plunger(s) may not be returning as have gone past the seal, try this first and change fluid at same time , cheapest option , when driving does the clutch slip ? if not would think clutch assembly is ok

If you have the non adjustable clutch push rod, the adjustment proceedure is to remove the clutch rod arm from the clutch shaft under the dash and reclock it so it pushes the clutch rod in farther...also check to make sure the master cylinder is full of dot 3 fluid and the system has no air in it.

hi it seems that you have right and the problem is the clutch master cylinder that is faulty if the leaking cylinder is located under the dash. the slave cylinder is at the gear box. replace the master cylinder and blead the system and you be alright. Hans

you need to bleed the system to gain the pressure back

It's possible your clutch fork broke. Also, check to see if it's a hydraulic clutch. You could have a leak in the hydraulic system. Both would cause your clutch pedal to go to the floor.

check the mechanical leakage at the clutch housing for binding and wear.