Firefox Could Not Install The File At – Unexpected Installation Error – 203 – FireFox Unable To Install Addon Due To Firewall

Trouble:

Some days back I suddenly noticed that firefox has started giving errors when ever I try to install the addon which is a small sized .xpi file. I tried many times to install the addon but none of the methods worked. I looked at the error console log in firefox which has following error message:

Error: [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsISafeOutputStream.finish]" nsresult: "0×80004005 (NS_ERROR_FAILURE)" location: "JS frame :: file:///C:/Program%20Files/Mozilla%20Firefox/components/nsExtensionManager.js :: closeSafeFileOutputStream :: line 508" data: no]
Source File: file:///C:/Program%20Files/Mozilla%20Firefox/components/nsExtensionManager.js
Line: 508

This type of problem generally occurs because of the corruption of extension files in firefox or there could be some firewall installed on your computer like ZoneAlarm or CA Firewall which is blocking the addon to get installed. 

Fix:

If the above is occurring due to firewall blocking the addon then you just need to temporary disable the firewall when you are trying to install the blocked addon or xpi file.

In firewall is not the case with you then you would need to delete the following extension files from the Mozilla firefox profile folder, make sure to Exit Firefox completely, then open your Firefox profile folder (read the linked article for its location) and delete the following 3 files
extensions.ini
extensions.cache
extensions.rdf

Find your profile in windows

Each profile is stored on your hard drive in a profile folder. The following table identifies the default locations of Firefox profiles on various operating systems.

The following examples refer to the default profile. The corresponding profile folder is named xxxxxxxx.default, where xxxxxxxx represents a random string of 8 characters.

Profile Folder Path Format

Windows Vista/XP/2000
%APPDATA%\Mozilla\Firefox\Profiles\xxxxxxxx.default\

Windows 98/Me
C:\WINDOWS\Application Data\Mozilla\Firefox\Profiles\xxxxxxxx.default\

Author + Admin: Rohit Khurana
Software Engineer by profession. I am a part time tech blogger and believe in blogging because I enjoy writing on technology. I also write about Cars and Bikes at www.carblogindia.com. You can catch me on twitter @rohit_khurana. Get in touch with me at RohitKhurana.in.