How can run appropriately in Firefox (mac) a web site designed just for Internet Explorer?

no... It has to be coded to use another browser, such as Firefox, if it isn't in the code then it won't work...

There is an add-in for Firefox that allows thepage to render as if it were running in IE at least for Windows. But even that is not foolproof especially for websites using Java.

The problem boils down to Microsoft has not been following the standards and people that code based on Micosoft create a site that only works with IE. Site operators and corporate amaerica don't care about supporting the alternative browsers.

In futere releases of IE, I heard that Microsoft is planning to follow the rules. Their fix supposeded will be render in a previous versionof IE.

Unfair? Yes. Creating a monopoly? Of course.