Thanks for the replies.
Tannin: Thanks for confirming that Firefox is using the monitor profile. It looks as if my approach makes sense. Phew.
Anyway, a few thoughts follow in case anyone is interested.
I think I know what Tannin means by input and output profile. I suspect most people understand the latter, but I find the former confusing.
As I understand it, an output profile compensates for errors in the output devices display of colours, so that it can display the image accurately. In practice it is a simple look up table used by the display card to map one value to another. Apparently look up tables are usually 8 bit, and in any case Windows XP only sends 8 bits per channel to the display card.
I think by input profile Tannin means the colour space. I have never really understood the underlying reasons for a colour space, although I have been thinking about it this evening, and I think I have figured it out. I suspect that it was introduced for efficiency, and the limitations of input and output devices.
In a bitmap each pixel is described as three numbers, one for each channel i.e. red, green and blue. The value of each number indicates the intensity of a component e.g. red. However representing a colour in this manner is rather wasteful, as many of colours cannot be captured or represented by current devices. Why include colours that we never use. So I suspect that the colour space was introduced as a way to represent colours within a restricted device specific range, or gamut. This has the advantage that colours within the gamut can be represented more accurately for the simple reason that the points corresponding to colour values are crammed together more densely. (That is also one reason why some people claim sRGB is superior to Adobe RGB despite having a smaller gamut.)
I believe that in practice a colour space represents each colour as a linear combination of three vectors. In a perfect world the three vectors would be at right angles, and would correspond to red, green and blue i.e. all colours. In 8 bits each component e.g. red would vary from 0 to 255. In a real world colour space such as sRGB I assume that the vectors are not at right angles.
So if the above is correct, the input and output profiles are quite different beasties. The input profile is just a way of representing colours in a restricted gamut. The output profile is a way of correcting for the deficiencies in a given display device.
So I guess that by converting an image to the monitor profile within PS is simply a way to ensure that you are editing the image as it will be seen when viewed in an application that does not understand input profiles. It is in effect a bodge for the fact that many applications do not understand colour spaces.
Anyway, I suspect I am not alone in being confused by this.
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