Negative friction force, positive normal force I realize that formulas don't always tell the whole story, but there seems to be some information missing from the equation: $F_f=\mu F_N$
Imagine we say up and right is positive. Then if an object is moving across a surface to the right, the friction force should be negative (acting against the positive applied force), and the normal force should be positive (acting against the gravitational force).
Let's say: $F_N=882\ \text{N}$ and $\mu=0.600$, then: $F_f=0.600\times822=529\ \text{N}$, which is positive, when it should be negative.
I realize that this formula converts between axes, so there can't be a reliable way to relate the signs, but then what is the convention? Does this formula just provide an absolute value for friction, and you can set it positive or negative, depending on the situation?
 A: It would probably be wiser to state the friction law as:
$$|F_F|=\mu |F_N|$$
where $|F_N|$ denotes the modulus of the Normal force.
Now consider the following diagram:

Both blocks and slopes are identical.

*

*Left: some net force on the block causes an acceleration $a$ (left and up). The friction force $F_F$ points in the opposite direction: it opposes relative motion.

*Right: some net force on the block causes an acceleration $a$ (right and down). The friction force $F_F$ points in the opposite direction: it opposes relative motion.

So the friction force opposes relative motion between the sliding surfaces.
A: In the relation $F_f = \mu F_{N}$, $F_f$ and $F_{N}$ are magnitudes of the frictional force and the normal force, and they are both positive. (I assume you are considering kinetic friction.)
The direction of the frictional force is in such a way that it opposes the relative motion between the two surfaces. This cannot be inferred from $F_f = \mu F_{N}$.
A: After thinking over the answers and discussion, I believe that the problem is just that using sign to denote direction only really holds up in one dimensional problems (unless you're using complex numbers). Therefore, the solution is to use the formula $F_f=\mu F_f$ to deal with magnitude only, and if using sign notation, you'll just have to apply the sign to the resulting magnitude as it makes sense for the scenario.
Update
After thinking on this a little more, the arrow notation isn't really any better for solving this situation. You'll be stuck with the same decision of applying the arrow based on direction of motion.
I think what I didn't make clear in my original question was that I wasn't asking for clarification about the situation, I just wanted to check on best practice / convention, since I am unexperienced.
A: No, the minus sign is not used to show direction.
A vector is just a direction and magnitude and cannot be negative. But it may be subtracted from another vector. This minus sign does not show direction but just a mathematical procedure. 
But it turns out that subtracting a vector $\vec A$ form another vector $\vec B$: $\vec A-\vec B$ is the same as adding the vector $\vec A$ to the reversed vector $\vec B$: $\vec A+(-1)*\vec B=\vec A+\vec B_{reverse}$ 
Thus is fundamental (and pretty obvious), but this is where the issue starts. The minus sign is not defining a direction of the vector, but if it appears in our math it cab be interpreted to flip a vector around. 
In general in physics, especially with forces: If you ever happen to get a minus sign in a result of the magnitude of a force, it just means that the vector is flipped around compared to whatever direction you defined to begin with (if that direction somehow is included in the calculation). If you do not get a minus sign, it doesn't mean anything at all about direction. 
