YouTube log-in link...FF4 & 5 refuse to go to that link, never a problem before

Question

Main YouTube page has a log-in link. Clicking the link to go to that screen invokes a bar at the top about switching to another page. I click OK but it dumps me back to the initial screen every time, refuses to go. I've tried starting with all plug-ins disabled, doesn't fix it.
I had the same problem with USPS tracking in their link to ask for e-mail notification, just wouldn't go there, so had to use IE for that. As of today, the whole USPS tracking site has changed, so that is no longer a problem.

Answer 1

-> Tap ALT key or press F10 to show the Menu Bar
-> go to Help Menu -> select "Restart with Add-ons Disabled"
Firefox will close then it will open up with just basic Firefox. Now do this:
-> Update ALL your Firefox Plug-ins https://www.mozilla.com/en-US/plugincheck/
-> go to View Menu -> Toolbars -> unselect All Unwanted toolbars
-> go Tools Menu -> Clear Recent History -> '''Time range to clear: select EVERYTHING''' -> click Details (small arrow) button -> place Checkmarks on '''Cookies, Cache''' -> click "Clear Now"
-> go to Tools Menu -> Options -> Content -> place Checkmarks on:
1) Block Pop-up windows 2) Load images automatically 3) Enable JavaScript
-> go to Tools Menu -> Options -> Privacy -> History section -> '''Firefox will: select "Use Custom Settings for History"''' -> REMOVE Checkmark from '''"Permanent Private Browsing mode"''' -> place CHECKMARKS on:
1) Remember my Browsing History 2) Remember Download History 3) Remember Search History 4) Accept Cookies from sites -> select "Exceptions..." button -> Click "Remove All Sites" at the bottom of "Exception - Cookies" window
4a) Accept Third-party Cookies -> under "Keep Until" select "They Expire"
-> REMOVE CHECKMARK from CLEAR HISTORY WHEN FIREFOX CLOSES
-> When using the Location Bar, suggest: select "History and Bookmarks"
-> go to Tools Menu -> Options -> Security -> place Checkmarks on:
1) Warn me when sites try to install add-ons 2) Block reported attack sites 3) Block reported web forgeries 4) Remember Passwords for sites
-> Click OK on Options window
-> click the Favicon (small drop down menu icon) on Firefox SearchBar (its position is on the Right side of the Address Bar) -> click "Manage Search Engines" -> select all Unwanted Search Engines and click Remove -> click OK
-> go to Tools Menu -> Add-ons -> Extensions section -> REMOVE All Unwanted/Suspicious Extensions (Add-ons) -> Restart Firefox
You can enable your Known & Trustworthy Add-ons later. Check and tell if its working.