Light to medium stain on fir will leave the grain visible. Fir darkens with age but doesn't blacken... Sometimes it actually gets more red with age. The best way to get an idea of what you want is to actually stain some scraps in colors close to what you want it to look like. You may know someone with a few cans of stain from old projects... I know I have about 12 different colors out in my shop Brush it on and wipe off the excess with a rag. I would never recommend a stain/poly combination because it's harder to work with and you won't have as much control over the finish. A water base or an oil base is fine as long as you let the water base dry completely. Oil seems to work better for me because it's not as likely to raise the grain.
When you do seal it with the clear coats, be careful when sanding between coats. A very light sanding will be fine between the 2nd and 3rd coats. Your first coat may not need sanding at all because fir will soak the first coat right in. Thin coats are less trouble and easiest to work with. You're not trying to fill the grain with the first 2 coats. I use Minwax, because it's widely available but when I need a custom matched color, I usually go to my Sherwin Williams store. They will try anything to get a perfect match even if it means telling you to do things like using a sanding sealer first, sanding lightly again, then staining. Maple and pine are best sealed before staining because they can come up blotchy with really ugly dark spots.
Either way, try your stain on a scrap first. You'll be way ahead by knowing exactly what to expect.