Thank you for contacting us. The Firefox browser extension is not functional at this time, but we are in the process of fixing it. We plan to have the fix available when we release version 6.1, which is just around the corner. We're doing everything we can to release that new version before the end of the year. For now, the only option for browser extensions is either using Mac OS Password Autofill in Safari or switching to Chrome temporarily.
mSecure 6 running on my Macbook indicates under the Preferences pane a button to install a plugin for Firefox, but the button does nothing. And, when I checked Firefox Addons, there is no listing at all for mSecure.