Gives error The requested URL "/", is invalid but I gave a valid format

Question

I gave firefox 3.6.6 what I think is a properly formatted URL, namely http://www.3dfx.com. Firefox changes this to http://www.3dfx.com/ I think the added / indicates it got a response from the URL I gave but then it says The requested URL "/", is invalid.
Reference #9.eb4a595.1295368725.9366789
so it seems like it did not understand the format. By comparison, I enter http://www.purdue.edu and it is changed to http://www.purdue.edu/ and the page is displayed.

Answer 1

Here's a (maybe THE) solution, assuming you have the HTTPS Everywhere add-on installed. If not, then this will probably be of no help at all (sorry).
I was having this exact issue, only when attempting to reach the Washington Post website, which had always worked for me without problems before. I received this error message:
The requested URL "/", is invalid.
To make matters even more confusing, I could reach the desired web site (using the same URL--http://www.washingtonpost.com) on the same PC using Internet Explorer, and on another PC using Firefox.
After an annoying search, Googling this error message, and trying a variety of suggested remedies, I was finally able to resolve the problem by changing the settings for the Firefox add-on HTTPS Everywhere.
In Firefox, click on the Tools menu, then select Add-ons, then click on HTTPS Everywhere and click on the Options button. The HTTPS Everywhere Preferences screen will appear asking, "Which HTTPS redirection rules should apply?"
Uncheck the website you're having the problem with and click OK. That should solve the problem. At least, for me it ended the problem immediately, without restarting Firefox, my PC, my router, or any of the other "remedies" I had tried during my earlier troubleshooting efforts.