Avast! & Firefox hate each other!

[REDACTED]

Windows 8.1 64-bit
Avast Internet Security v. 12.3.2280 (build 12.3.3154.0)
Mozilla Firefox 48.0.2
Windows Defender turned off
No other security features active

I have Avast! IS working with Firefox on my Windows 10 machine, but my daughter's laptop (stats above) is giving me fits. Every single time I try to use Firefox on her machine, I get this message, no matter what search engine I try to use:

"Your connection is not secure."

I did some searching on this forum and other places, and I went into the Avast! IS Settings>Components>Web Shield>Customize, where I unchecked the "Enable HTTPS scanning".  Then I saved everything, and restarted the computer to be on the safe side.  Upon restart, same error from Firefox. 

I uninstalled Firefox.  Restarted.  Re-installed Firefox.  Restarted.  Initialized Firefox- "Your connection is not secure."  Tried multiple search engines within Firefox, because, what the hell, why not?  Google, Yahoo, Bing, Dogpile- "Your connection is not secure."

Internet Explorer works, but ewwww.   Avast! SafeZone works, but it's clunky. 

Is there ANY way to get Avast! IS and Firefox to play nicely together on a Windows 8.1 machine?  Please keep in mind, I'm a novice at tech stuff, so be gentle! 

[bruce_b]

Try and delete this file: cert8.db ... it should be in your Firefox profile ... Firefox will recreate it when it is opened again. Firefox must be completely closed before you can delete it. Also clear all the Cookies in Firefox and check Tools, Options, Advanced, Network, Setting and be sure it is set to No Proxy or Use System Proxy 

This link will tell you where to find your Profile Folder ... http://kb.mozillazine.org/Profile_folder_-_Firefox