In the Location bar, type about:config and press Enter.
       The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise! to continue to the about:config page. 
   In the Search textbox, enter xpinstall.enabled.
   The value of xpinstall.enabled should be set to true. If this value is currently set to false, then software installation is disabled.
   To set the value to true, right-click the property name, and select Reset.
   Try to re-install the extension. 

I am having a problem installing an add-on for Firefox on my XP desktop: When I attempt to install Evernote Web Clipper 5.9.1, I get the error: "There was an error installing Evernote Web Clipper - Try Again". I investigated on the Firefox Help site and found an instruction (copied below) to look in the Firefox "about.config" file to see if the "xpinstall.enabled" listing was "true" or "false". Supposedly, if the listing is "false" it should be changed to "true" to permit installation of add-ons. Software installation disabled It is possible to disable all installation of software, including add-ons, through Firefox. To check to see if software installation has been disabled: In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise! to continue to the about:config page. In the Search textbox, enter xpinstall.enabled. The value of xpinstall.enabled should be set to true. If this value is currently set to false, then software installation is disabled. To set the value to true, right-click the property name, and select Reset. Try to re-install the extension. There is no "xpinstall.enabled" listing in my version of "about.config". I have: xpinstall.whitelist.add user set string xpinstall.whitelist.add.180 user set string xpinstall.whitelist.add.36 user set string The spaces for "value" on all three listings are blank (empty) What to do?