Firefox could not install the file npractrl.xpi

To manually install the plug-in ( and can be installed only in windows version of FF ).

Alternate Method:
If this method doesn't work then :
1. Backup your profile folder.
2. Goto start | control panel >Add-remove programs.
3. Uninstall FF.
4. After that remove FF program folder completely.
5. Download fresh firefox installer from : "http://www.mozilla.com/en-US/firefox/".
6. Install it.

Primary Method:
1. Change .xpi to .zip
2. Extract files from it.
3. Copy these files to 'plugins' folder ( located within Firefox program folder ) :
'npRACtrl.dll'
'ractrlkeyhook.dll'
'unicows.dll'

4. Copy 'npRACtrl.xpt' file to 'components' folder
5. Rename 'pluginreg.dat' file to 'pluginreg.dat.1' ( located in FF profile folder
"http://kb.mozillazine.org/Profile_folder#Where_is_my_profile_folder.3F" )
6. Start FF