I believe as it stands that 80% of the world uses Internet Explorer 6. I don’t recall exactly how long it has been, but Internet Explorer 7 has been out for a while as well. Firefox is an amazing browser as it follows standards, and they will be releasing a new version later this year. Internet Explorer is also working on their next version. One mustn’t forget that Opera and Safari also exists too.
There is a sea of browsers and I’ve only mentioned the mainstream ones. For all I know there are numerous others that a small niche group are using. The problem with all these browsers is that each browser has its own set of rules and with that, webpages display different in each. As a design firm, all I can say is, it is one of the most aggravating things to deal with. You can have something that displays perfectly in IE and FF but when you open it up in Safari it looks like you threw up the design on the page.
Unfortunately, the only solutions are that everyone start using the same browser or the companies developing the browsers get their butts in gear and start following the same standards. Sorry for the rant, but I just needed to vent that I am not looking forward to a new IE, unless they standardize it with Firefox. By the way, if you are tired of IE like most people are, Firefox is amazing.