One of the hardest tasks for every single web designer and developer is making their site to look a certain way in all web browsers and operating systems. It’s hard because every single browser uses its own different render engine.
You should remember that not everybody is using the latest Firefox version. Even if your code is clean and semantic and the site looks good in Firefox it could be total mess in Safari, Google Chrome, Opera or Internet Explorer (especially in Internet Explorer)
It’s a must that every web designer and developer has all the major web browsers ( Safari, Google Chrome, Opera, Firefox) installed on their computer . If you have them all don’t start to celebrate yet, you still have a problem. You are probably asking your self “A problem , what problem?!? “ Internet Explorer (IE) of course . It’s the inbuilt Microsoft browser and there are still lots of people who use IE version 7, 6 and even 5.5 (yes I have visitors with Internet Explorer 5.5 on my blog). The IE Tester is the perfect software for you know, because it supports versions 5.5, 6,7 and 8 at the same time.
On the other hand why the hell you should support 8 years old browser or even the older Internet Explorer 5. Changing the CSS because of these browsers is just waste of time. Microsoft finally included Auto Update in IE7 and the market share of IE 7 will go down fast 
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Ultimately you should know your audience extremely well and you should decide which browsers to be supported in the current project.
There are web solutions for cross browser testing which you can use if you don’t want to install 5, 10, 50 browsers on your PC. The one I am using is the site Browsershots .
Note: Your site should look well in all the major browsers, no one said that it should look exactly the same