While I have just found a very useful application (or plugin, I should say), it's only available for FireFox. And that's the only thing stopping me from downloading the plugin.
Are there any tools just as good as, or better than FireBug for FireFox that are not browser plugins? And if so, could you please kindly share them?
You can also get Firebug Lite, which gives you the power of Firebug in any browser. You don't even have to download it, just add a
<script> resource to the page you want to debug which points at the hosted version of the debugger.
Whichever tool you use, debugging 500 lines of 4 year old script is going to be a huge nightmare. Because the browser market has changed so much in this time a lot of the JS used is probably innefficent. It may be quicker and easier in the long run to just re-write it from scratch using current JS best-practice...
Firebug is my favorite. Safari and Opera also have integrated debuggers (that I don't really use, so now sure how good they are). For IE, you can download the IE Developer Toolbar (although the debugging is not great, but it's better than nothing). If you have access to Visual Studio, that's the most powerful tool for debugging IE (you can hook into the browser from within VS). I'm not sure what you have against Firefox/plugins, but honestly Firebug is the way to go unless you are debugging IE-specific issues.
Why use FireBug
Because it tends to be the best. Most developers will tell you that this is the universal truth (event though the latest combination of FF anf FB seem to be a bit buggy). You probably won't debug on various different browsers anyway. You'll priobably debug on one and make your code cross-browser compatible.
But if you want to debug on all, you can. There's a very similar debugger available in Chome (V3). It's built in so you don't have to do any plug-in downloads. The same goes for IE8. They improved their developer tools considerably with IE8. So debugging on all three became a very similar experience.
Keyboard shortcut to debuggers in all browsers
You can access debuggers/tools on all browsers using the same key combination F12.
Why in-browser debugging tools are the way to go?