Internet Explorer, created by microsoft has been most popular web browser for many years. But gap is shrinking with release of Mozilla Firefox, by an open source community.
At last count it is said that there are 64 million firefox users on internet. Growing in massive numbers by day.
So, issue with browser compatibility is at its highest importance. The way browsers are constructed, they can show a webpage slightly differently.
For instance, IFRAME tag shows perfectly in Internet Explorer but does not show in Firefox. This is only one of many instances of none browser compatibility.
Therefore, webmasters should be making sure their web page is viewable in both internet explorer and firefox equally. If it doesnt, then they risk loosing a large percentage of web users. There is nothing worse than surfing a web site and not being able to see it properly in a particular browser.