You should source it from a non-coastal or salt-soil area because it's very adept at keeping salts locked into the bark.

That said, if you get good winter rainfalls and you're not planting in it during the winter it should be okay by spring planting season no matter where you source it.

It is organic matter and organic matter is good for the soil. As mulch it is quite good for aiding in suppressing unwanted plant growth, aiding in moisture retention, aiding in adding organic matter to the soil, slowly, and aiding in soil temperature control. I see no downside to using Fir bark as mulch.

I've seen situations where too much bark can turn into a water-repellent anti-sponge.

Wood chips are a better weed barrier, and generally don't glue up like bark can. In some places, you can get all you want for cheap or free. As wood chips break down, you get a really healthy fungal mat for the soil. If I'm really worried about appearances, I throw a thin layer of bark on top of the chips.

Don't let mulch get stirred into your plants' mineral rootzones, or you'll mess up the C:N balance in the soil until the brown stuff gets eaten down into black.