For your information, I too have Firefox installed on my PC, along with two other little known web browsers called Deepnet Explorer and Hydra. Anyway, I normally use Firefox if I am just surfing around from here and there, because I do agree that Firefox is a faster browser than Internet Explorer. But when I need to do some risky thing like transferring money from my bank account to my IRA account, logging in to my Scott Trade account's trading console or just booking an airline ticket, I will then use Internet Explorer.
For example, I was recently applying for Social Security disability benefits on behalf of an older friend of mines online about two months ago, but this federal government website made it mandatory the use of Internet Explorer! I tried using Firefox and it gave me an error. You see, these so called "badly written, malformed scripts" as you put it, are actually the norm in much of today's business, government and to some extent, entertainment websites. And the more flashier AJAX scripts, they are actually part of the W3C standards for "Web 2.0". Google it and see for yourself.
There is this website about cutting edge DHTML scripts located at: www.dynamicdrive.com It is perhaps the best bank of Dynamic HTML scripts on the net. Using Internet Explorer, I viewed the working live samples of just about all the scripts on the site with a 100% success. I will like you to try that using Firefox, to see what will be your success rate. After that, you can go ahead and tell the folks at Dynamicdrive.com that some of their scripts are badly written, I'm pretty sure they would like to hear that. Firefox is a better browser in the speed and to some extent, HTML and CSS display. But it needs to do some serious catching up on the scripting and encryption departments, and it's not me saying it, it's internet establishment saying it.
For a more further and deeper discussion on all of this with experts in the field, I recommend you checkout: www.programmingtalk.com