Firefox Still Cannot Install Extensions by Default

As previously stated, the error that Firefox reports back is an unknown error, which really doesn't help us. What is of value is the information after this error report. To be precise, the error is shown in a popup. The popup for either Firefox 2 or 3.0 b5 says something to the effect of:

Firefox could not install the file at

https://addons.mozilla.org/...../googlebar-0.9.15.11-fx+nx.xpi

because: Unexpected installation error
Review the Error Console log for more details.

-203

For Firefox version 2, when you go to view the Error Console you might see something similar to this:

installLocation has no properties
file:///usr/lib/firefox/components/nsExtensionManager.js

For the beta version, when you go to view the Error Console you might see something similar to this:

installLocation is null
file:///usr/lib/xulrunner-1.9b5/components/nsExtensionManager.js

For both of these errors if you look to the right of them you might see what line number they're located at in that file. As you might be able to surmise, simply edit that file with your favorite editor after this line(3738 or 3938 depending on the browser version):

var installLocation = this.getInstallLocation(item.id);

and insert this line immediately after it:

if (!installLocation) continue;

Save the file and restart Firefox. At that point you should be able to install your add-ons.

I'm not saying this is guaranteed to work for you, what I am saying is that it worked for me. This patch was found at the bug's launchpad site amongst the user comments. If you view that bug and what the users had to say, you'll be able to tell that this patch might not work for everyone, but that page contains other fixes that you could try if you still have problems.

At last, my add-ons are installed and everything is fully functional in both version 2 and the beta version... for now.