First port of call should be the built in tools you have available to you.
I see that you are using Dreamweaver from your page source code. Open your web page in Dreamweaver then click on File, go down to Check Page then select Check Target Browsers. You can select from a list of previous version of IE, Netscape and Opera but oyu won't like what it tells you.
Netscape and Opera have some unusual ways of interpretting code and sometimes don't even copw with plain vanilla HTML tags properly. In fact, there is a list of code tags a mile long that Netscape either does not support at all, does not recognise or only partially copes with.
Welcome to the headaches of web design, where your creation falls to bits under scrutiny by any other browser than the majority share IE. This has been a major headache for as long as I can remember and you'll get a real scare if you start trying to rework your pages to fit allcomers in the browser world.
You can also nip some of your problems in the bud at design time by clicking on Edit, Preferences then select Validator and check the IE4 and Netscape boxes so that when you validate your code (File, Check Page, Validate Markup) any issues with the browsers you've chosen will be highlighted for you.
Try the Dreamweaver tools to begin with for some nudges in the right direction.
It will open your eyes at the very least.