Firefox won't open websites, other browsers do

Question

So, any website that I try to open up in Firefox just won't load, it's not just particular websites. But all other browsers - IE, Chrome and Opera, do.
What I have tried:
* Checked Comodo's firewall about Firefox, didn't see anything
* Reinstalling Firefox
* Flushing DNS
* Checked Firefox proxy settings
* Resetted Firefox to it's original state on "about:support"

I'm out of ideas now and fast googling wouldn't give me any good answers.

Thanks for helping!

edit
OS: Windows 7
Internet connection: Wired, 12/1
When/how did this problem start: The person who I'm helping does not remember that he had done something that might've caused the issue.

Solution

You can try creating a new profile following the official instruction here: Use the Profile Manager to create and remove Firefox profiles.

A new profile will not contain previous settings, while re-installation of Firefox may have kept the old settings that were messed up. I'm not sure whether it has the same effect as "Reset" in about:support.

Comments

[Daniel R Hicks]

What's your operating system? What kind of internet connection? When/how did this problem start?

[Sun]

1) Have you tried Firefox Portable? Can you get to web sites? 2) Rather "reinstall", I would suggest you try uninstall, then install a fresh copy of Firefox. There is a difference especially if there are corrupt files that are not being replaced.

[Soren]

Do you have any ad-blocker installed in Firefox?

[Wes Sayeed]

Please summarize the content of external links so future users do not have to follow them to get the gist of what you're saying.