Firstly, yes JavaScript is enabled!

Here's a list of all related modules I have installed:

• Token, uBrowser, Tables API
• Cart, Order, Product, Store
• Notify, Payment, Shipping Quotes, Reports, Shipping
• U.S. Postal Service, Weight quote
• PayPal
• JQuery Update 5.x-1.0
• Workflow-ng.*

I believe I've provided all information for each of the modules, including a default store address, product weight, and USPS user ID.

I've got Firebug installed, but am not quite sure which of its many panels of information I should be looking at, nor what I should look for. Here's what shows up under "Console: Post"

details[city] Pasadena
details[company]
details[country] 840
details[first_name] David
details[last_name] Barlia
details[phone]
details[postal_code] 91101
details[street1] 380 Parke Street
details[street2] Unit 8
details[zone] 12
products 6%5ETesting%5E98709870987%5E%5E2%5E0.00%5E1.37%5E0.125%5Ea%3A2%3A%7Bs%3A6%3A%22module%22%3Bs%3A10%3A%22uc_product%22%3Bs%3A9%3A%22shippable%22%3Bs%3A1%3A%221%22%3B%7D
uid 1

And the "Console: Response..."

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This error comes up when I first go to the checkout page:

[Exception... "Component returned failure code: 0x80070057 (NS_ERROR_ILLEGAL_VALUE) [nsIFileOutputStream.init]"
nsresult: "0x80070057 (NS_ERROR_ILLEGAL_VALUE)"
location: "JS frame :: file: //C:/Program%20Files/Mozilla%20Firefox/components/nsSessionStore.js :: sss_writeFile :: line 2055"
data: no]
file: ///C:/Program%20Files/Mozilla%20Firefox/components/nsSessionStore.js
Line 2055