Same failure occurs with Firefox 53 and ActivClient x64 version 6.2.0.174. Rolling back to Firefox 52 is not an option due to security patch requirements. However, Firefox ESR 52.1.2 works and meets patch requirements.

Also see: https://support.mozilla.org/en-US/questions/1163019

Based on Eng78's reply I downloaded and installed 52.2.oesr and that fixed the problem. I have also confirmed this is a problem with Firefox 54.0 as well as the 53.0.x versions, both 32 and 64 bit. Glad the esr version fixed the problem but worried it will pop up again in a later update to the esr versions.

There is a reason this is happening. You will notice that in the Menu/Options/Advanced/Security Devices, there will be the name of the card reader that you use for the Common Access Card (CAC). When you click on the reader, the status must show "Logged In". If the status does not show that, click the button on the right side of the window to login. This will prompt for the CAC pin. If you do not login your CAC in the Firefox browser upon every launch, the browser will hang at a CAC enabled website. If you have the luxury of rolling back, 52.2.oesr will fix this. However, most government systems require to be on the latest version and this is not an option. Until Mozilla figures out a way to prompt login for a security device on browser startup, this process is necessary. I hope this helps some of you.