Try starting Firefox in safe mode. It may be a rogue add-on. (https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode). You might also want to check "about:config" sort by status/user, search for the bookmarked website or the one it redirects to. If a result(s) pops up right click then select reset. After that restart Firefox and see if it fixes your problem.

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 ("History > Show All History" or "View > Sidebar > History") 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 sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of the involved 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.

Nice tip cor-el...I had never looked at this. Any thoughts as to how this occured? Malware? I have never had a problem and do not use any security software. Should I? I am on OS X 10.9.5

Thanks again.