Firefox reports invalid Security Certificate but doesn't allow you to add an exception

By Selwyn Polit, 25 August, 2021

My beloved Firefox has been doing something strange when it tries to view websites that have certificate problems such as expired certificates.  For some reason my desktop computer would pop up a bland dialog box whining about the site uses an invalid security certificate.  My laptop doesn't seemt to do this.  Apparently no-one on the internet has ever experienced this problem so I felt it would be useful to share the solution with folks. 


Here is the alert box:

Thanks to a lovely person who goes by cor-el on the mozilla support website, I got the following solution which I am translating so it will be comprehensible to mere mortals.  (For Techies: You should set the browser.xul.error_pages prefs to true on the about:config page.)

Essentially it involves the following steps:

  1. Type about:config into firefox where you would normally type a URL. (see screen shot)
  2. type browser.xul in the Search box
  3. double click on the line that says: browser.xul.error_pages.enabled and making certain that the Value is true. 


When Firefox is configured correctly (Don't ask me how it became unconfigured..) the dialog looks like this:


This peson also kindly offered a little more information about how to fiddle with the about:config options in firefox.  Be gentle - you can break some stuff.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

And while we are here, you can find the stored certificates at Tools / options / advanced tab / view certificates / servers tab.