I have finally worked out what is happening to cause this problem, but only in Firefox.
For this site it was that the minimum font size had been set at Verdana 14, forcing the browser to adjust to a fixed size font regardless of the site designer's intended layout.
However for another page I was having a similar problem with, removing the minimum font size made no visible difference.
For Nine MSN News Australia for example, the issue was caused by a Firefox extension I use called Redirect Remover.
So as you can clearly see the problems on each site may or may not be related to each other, making tracking them down infinitely more difficult than than it might have been if there was only one single problem causing all of them.
What's new?
I am continuing to explore each problem page in turn, but at least with reference to the Mystic Scripts Online library and the appearance of the graphics on Mystic Boards, the problem seems to have fixed at least for now.
What might happen when I eventually update to Firefox 4x (they are now up to Beta 12) is anybody's guess, so please stay tuned.
Thanking you for your patience and help.
EoT
PS: I have since gone back and deleted all my JPG attachments, in order to reduce the space required to store this thread in the forum database.