I guess you are using 32 bit Non-Qubes-Whonix and installed 64 bit
software. Through an undocumented way.
Generally, use apt-get. Only if there is no alternative and a strong
reason do manually install.
Firefox is called iceweasel in Debian and already installed from Debian
package sources.
But you should not use firefox / icewesel in Whonix anyhow. Use Tor
Browser. Why and documentation:
See also: