There are 3 common reasons to get SSL certificate errors on numerous sites all of a sudden:

• Error in your computer's system clock, either date, time or time zone
• New or updated security software that filters your web traffic, or Firefox Reset and you have such software
• Malware infection

It's easy enough to check the clock.

To distinguish the other two issues, you need to inspect the certificate that Firefox is rejecting. There is a trick to this, which is to expand the third section of the error page ("I understand the risks") and click the Add Exception button. If Firefox doesn't enable the View button click "Get Certificate". Once you can click the View button, go ahead and click it to launch the certificate viewer. There, check the "Issued by" information. Often this will point to the reason that Firefox objects to the certificate. Please post what you find in a reply. You don't need to finish adding an exception, you can just cancel out.

Or... maybe... have you installed the HTTPS Everywhere extension? Because if you follow the link to

http://petperks.petsmart.com/

it redirects to

https://www.petperks.com/

But that redirect does not work if you start off trying to go to

https://petperks.petsmart.com/

And an extension like HTTPS Everywhere which tries to make all connections secure can cause this problem.

To check and disable extensions, and often you can configure exceptions, too, open the Add-ons page. Either:

• Command+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions.