If you're talking about mailto: links, then Options > Applications and look for mailto and choose your email provider/application under Action

You have missed my point. I open my home page - igoogle, which contains gadgets for news articles from various sources, Each news item title shows a hyperlink to the article itself, Clicking on the hyperlink accomplishes nothing.

Only hyperlinks inside iGoogle?

The links that do not work in Firefox as a browser do work under IE.

They never worked in Firefox, or were working and stopped working after you updated to version 4?