Before configuring Firefox to display PDFs within Foxit Reader, make sure the following things are done:
1. Foxit Reader Plugin has been installed. Normally, it was installed by default. To check, look into C:\Program files (x86)\Foxit Software\Foxit Reader\plugins, see if there is a DLL file named as “npFoxitReaderPlugin.dll".
2. Associate Foxit Reader Plugin with Firefox. Open File > Preferences >File Associations > Advanced > check the option "Include browser when setting default PDF viewer", click on Ok to apply the change.
Two steps to configure Firefox to display PDFs with Foxit
In Firefox, go to Tools menu> Add-ons, find Foxit Reader from Mozilla and choose Always Activate .
Firefox add ons can be found under the menu tab, not the tools tab.
Check if there are any other PDF viewer plugins activate there, if any, choose Never Activate to disable them.
Go to Tools menu > Options > Applications, type PDF in the search text field, drop down the Action list, select Use Foxit Plugin for Mozilla. In the image below uses Foxit PhantomPDF Plugin for Mozilla as an example.
Instructions should be menu tab > options > applications.
1) If you want to disable the Firefox built-in PDF viewer entirely, type, "about:config" in the address bar, then click on the "I'll be careful, I promise!" button. Next, search for pdf js.disabled, then double click on the entry, which will turn the boolean value from "false" to "true."
2) Currently, supports 32bit version of Firefox only. To check your Firefox version number, type, "about:buildconfig" in the address bar, find the information below the target under Build Platform.
Note: NPAPI plugins are no longer allowed in Firefox 52 and above, so this solution only works for Firefox versions prior 52.