On 6/15 FF updated both of mine and my husband's computer to FF 13.0.1 and something called Mozilla Maintenance Service. I am only mentioning my husband's PC because his computer is also having the same error now and we have our own PCs and we both just had FF to pass on new updates to each one of us. I don't know if this problem started right after the updates because it was only on 6/19 I tried to access an online newspaper that I have a paid subscription to. At first I thought it was just my computer that could not get to this site, but then my husband tried to go to this same site with his computer and also received the same error message. This was the error message:
ERROR
The requested URL could not be retrieved
While trying to retrieve the URL: http://chronicle.augusta.com/
The following error was encountered:

Access Denied.
Generated Fri, 22 Jun 2012 01:13:23 GMT by sms2.cac001.groupz.net (squid)
Since neither one of us could access the site from 2 different computers, I thought it might have to do with the web site itself. So I waited until this morning and tried again--same results. Then I called the newspaper itself and they said that no one else was having this problem. My next thought was that it might be a ISP problem, that maybe our ISP was blocking us from access to the site. And a funny thing I noticed is that this web site also no longer showed up on my Face Book site (the head lines). My husband & I spent 2 hours on the phone with tech support with our ISP--he tried everything and nothing worked. Finally my husband had an idea and tried to access the web site with IE--and IE had no problem getting on the site. So then we knew it must be a FF problem--and both of our FF had just been updated a few days before.

Tonight I spend hours on FF web site doing research on their site and got over a 1000 search results. Some of the problems I saw that related to mine were people being denied access to different web sites, but most of them had to do with a much lower version of FF than mine. But still I read the answers to the solutions and tried them all--and nothing worked. The following is what was recommended the most to fix the problem and I also tried it:

"Clear the Cache":
Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
"Remove Cookies"
I not only cleared all the cookies, all of the history, all the cache, etc.--I cleared everything. But this still did not solve the problem either. These are some other suggestions that I tried: In the Proxy Settings under Tools, Advanced, Network Tab, I selected "No Proxy" even though we don't connect through a proxy. No help there. Next it was suggested that maybe a firewall was prevented access. The only firewall I have is Windows 7 Firewall, so I just disabled it to test and see if that was the problem--still could not access the site. Then I found a suggestion that said:
Firefox supports IPv6 by default, which may cause connection problems on certain systems.
So I followed the instructions on how to disable IPv6 in Firefox--still did not help. I have tried every solution suggested on FF web site that I could find and nothing has worked. My only thought left is that it has to do with updating to FF 13.0.1 and to Mozilla Maintenance Service on 6/15. So far I have not come across any more web sites I have been denied, but I have not been to all my regular sites yet since it has just started happening. I am really upset because I pay for a subscription to be able to read the newspaper online and am not able to do so because of this error.

This is the pertinent information about what is running in real time on my computer:
Windows 7
FF 13.0.1
SuperAntiSpyware
Avast Anti-spyware

I will be up for a little while longer and will check back here again before going to bed.