This is a known conflict with the latest updates to Bitdefender and Firefox; to resolve, please do this now:
Click on the Bitdefender icon on your desktop/system tray to bring up the main Bitdefender interface.
Next, click on Settings from the upper right toolbar.
From Settings Overview, choose Privacy Control.
In the Antiphishing tab, use the switch to turn OFF the Scan SSL feature.
Done that, re-open Firefox and check again - your issue should be resolved now.
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