I don't know if this is caused by Firefox or Foxit. Here's what I have tried so far:

1. Uninstalled and re-installed Foxit with all the plugins.
2. Disabled plugin in Firefox, restart, enabled plugin in Firefox.
3. Disabled the built-in Firefox PDF reader by going to about:config and changing pdfjs.disabled to true.
4. In Firefox went to Options-Applications and changed the default application to both Foxit 6.1 and also the Foxit Plugin.
5. Foxit is the default and only PDF applications installed.