In Fireworks and for an image similar to this:
what's the best way to horizontally distribute objects in their parent container so that they have equal margins? The same effect can be achieved with
margin:auto in CSS.
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You're looking for the alignment options that come up with the Move tool. The first 3/4 or so of this tutorial cover a situation fairly similar to what's happening in your image. Step 7, combining use of the "align vertical centers" and "distribute horizontal centers" buttons is the important bit. I couldn't find the specific reference for the tool in the mess of Adobe's docs, but you can give it a try.
You didn't mention what version(s) you're working with, but I think the alignment options were only added to Fireworks in CS4.
I don't know about Fireworks, but Photoshop is CLEARLY lacking in distribute options.
There is no inherent method for distributing space within Photoshop. All the alignment buttons will do nothing to ensure spacing is equal. Photoshop aligns by object bounds and at times those bounds can be so dramatically different (as with type layers) that no matter how often you click alignment options, nothing distributes evenly.
This is why some cleaver people have written scripts to distribute spacing.
Click this to find some links to scripts for distributing space within Photoshop. I use these on a daily bases with Photoshop CS4 and newer without any problems.