I know this sounds a little obvious, but in Photoshop, have you checked the mode (Image/mode) you are in ?
CMYK and LAB don't support PNG.
If you have IE set as your default browser (which I find hard to believe considering your relationship with FF) this would explain the Photoshop help file not be able to work, as it needs a browser to function.
The help file is just looking for the same browser thats not working.
Looks to me like the quickest solution is to re-install IE.
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