[CentOS] Firefox Issue

Everyone is back at work and starting to use computers on our
smallest network which has Internet access through a rather old
Linksys router.  Two systems were left on and screen-locked over
the extra long weekend.  There does not appear to have been any
Internet access interruption in our absence.

A Firefox browser on one system was left pointing to a commonly
used web site: https://www.yahoo.com/.  This Yahoo web page was
not displayed when the user unlocked the screen and brought up
the browser from the task bar.  

Instead, a site located at the link https://gaibacoupontec.com
was displayed with a message indicating that there was an urgent
Firefox update required.  There was a button to download and to
install the update.  I killed the Firefox browser rather than
getting rid of it with the X in the upper right hand corner.

This event has the aroma of an unwanted cyber intrusion, which
is why I killed the browser.  I have also copied and stored the
full URL displayed in the browser, but have only included the
first part "https://gaibacoupontec.com" here so as not to tempt
anyone to risk access.

Is it possible that a new Firefox flaw has been detected and is
being exploited for malicious purposes?