It's confusing that the forum software detected both 10.1.4 and 11.0.2. And that another page mentions a different version.

Which version of Adobe Reader or Adobe Acrobat do you actually have?

I have version XI of adobe reader and Acrobat X pro, actually I see there are both versions installed, 10.1.4 and 11.0.2. However I disabled 10.1.4 version but still does not work.

Could you review/adjust your preference for which program opens PDFs? I'm not sure how that drop-list behaves when there are multiple Adobe plugins. You might need to physically remove an old plugin file or two to make sure Firefox is not trying to use the old one.

• Use Adobe Reader to view PDF files in Firefox
  

Edit: actually this article describe how to use the Options dialog Applications section to review/change the setting: View and edit PDF files using Firefox’s built-in viewer.

• C:\Program Files (x86)\Mozilla Firefox\Plugins (only one want nppdf32.dll here)
• In regedit.exe, check the following key for the source of obsolete plugins: HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\MozillaPlugins

Both the file that stores download handling preferences, and the file which registers plugins, can be renamed and Firefox will regenerate them, so those remains options for further actions as well.