FireFox "Software installation has been disabled by your system administrator"

Question

Installing Firebug in FireFox I get this error message:

Software installation has been disabled by your system administrator

I am a administrator of this PC, how do I allow installation of Firefox addons?

_{Note: answering my own problem to help others in the future.}

Answer 1

The root cause is this setting:

This blog posted 3 years ago has a solution with Windows XP, the trick back then was to change this setting:

user_pref("xpinstall.enabled", true); in the file C:\Documents and Settings\UID\Application Data\Mozilla\Firefox\Profiles\fxlme5ka.default.

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To do this on a recent version of Firefox, follow these steps:

1. Close Firefox

2. Open the C:\Program Files (x86)\Mozilla\Firefox\mozilla.cfg file in Notepad

3. Comment out the rule that is blocking it:

// Block extensions
//lockPref("xpinstall.enabled", false); // prevent extensions

4. Restart Firefox