Do the following steps:
Type about:plugins in the address bar
Find the Adobe Acrobat section to get the following information:
File: nppdf32.dll, nppdf32.dll
Path:
C:\Program Files (x86)\Adobe\Reader 11.0\Reader\browser\nppdf32.dll,
C:\Program Files (x86)\Adobe\Reader 11.0\Reader\AIR\nppdf32.dll
Rename and move the following files out to a different location:
C:\Program Files (x86)\Adobe\Reader 11.0\Reader\browser\nppdf32.dll,
C:\Program Files (x86)\Adobe\Reader 11.0\Reader\AIR\nppdf32.dll
By clicking on the "Firefox Menu > Tools > Add-ons > Plugins", you will notice that the "Adobe Acrobat" plugin is successfully removed
