Please explain the problem in detail. What happens? What are the exact error message(s) ?

Suspicious connection blocked 23 hours ago

Feature: Online Threat Prevention

"firefox.exe attempted to establish a connection relying on an unmatching security certificate to aufp.io. We blocked the connection to keep your data safe since the used certificate was issued for a different web address than the targeted one."

This happens when I first fire up Firefox but not every time.

That's all I can tell you. I can unblock it if necessary. Just thought Mozilla would want to know.

Leinieman said

This happens when I first fire up Firefox but not every time.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.