Disable your Firewall and antivirus and give a try.

Disabling the firewall did not fix the problem.

which antivirus are you using?

Does this also happen if you enter that website via the main entry (i.e. remove all trailing path data from the link and only keep the domain)?

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry (Show All History or History sidebar) or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious and if you have a password or other data from that domain that you do not want to lose then make a note of those passwords and bookmarks. You can't recover from that 'forget' unless you have a backup of the affected files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

cor-el, The answer to your first question is, yes. I cleared the cache and lost all Firefox tabs that were currently open. I am clicking on a link (Help Contents) within a Mozilla program (Tbird), to take me to that page in another Mozilla program (Firefox) and I get a 403 Forbidden message. I believe that the problem WAS on Mozilla's end. I say WAS, because this problem is no longer present for reasons that I cannot explain. I believe that there have been issues with Thunderbird lately. Pat