Good quality old growth doug fir has some rot resistance. But not nearly as much as Red Cedar though it should be noted that here too the wood with the most rot resistance is the heartwood of older trees. (see below)
I think that fir may be a fine choice with the right detailing (dry good, wet bad)and if it is not exposed to the weather full time. It should also be noted that cedar has not nearly the strength of fir but in the case of posts which are frequently oversized for appearance this may not matter

Following is a quote from the Weyerhauser web site.

"What is the natural decay resistance of Western Red Cedar?[/B]

The excellent natural durability of Western Red Cedar heartwood is recognized throughout the world. Probably one of the most persuasive European research papers is by Bellmann. This paper is in German, and it highlights Western Red Cedar as being one of the most durable species in the marketplace. Western Red Cedar contains naturally-occurring water-soluble extractives than can leach out during service (one of the reasons why it changes to a grey colour over time). For this reason, it is suggested to use a natural oil-based clear stain to improve longevity of the exposed product.

How does the decay-resistance of Western Red Cedar vary with colour?[/B]

There is no direct correlation between Western Red Cedar colour and the extractives content of the wood. Rather, it is the radial location from where the board came from that typically signifies how resistant the wood could be. Therefore, darker cedar boards do not indicate that they could last longer in service. The white-coloured sapwood contains considerably fewer extractives, but can easily be distinguished, even beside the lightest coloured heartwood.

Second growth cedar also has a lower content of extractives. Research shows that it is not a result of second growth cedar "becoming a new kind of tree". It has been observed that Western Red Cedar trees accumulate more decay-resistant extractives as they get older (and In the case of second growth trees, the accumulation is at a faster rate). As of yet, there is no forest policy to extend the rotation for second growth Western Red Cedar trees in order to increase the extractives content in the wood. This policy may have to be reviewed in 30-50 years as second growth western red cedar begins to enter the market."