Did you try to type the address in the location bar instead of using a bookmark?

Reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

   Press and hold Shift and left-click the Reload button.
   Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
   Press "Command + Shift + R" (MAC) 

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.

Could you test these two links and see whether they work:

• http://www.google.com/ (not secure, may redirect)
• https://www.google.com/ (secure connection)