This usually happens if the extension needs to be unpacked in order to work properly. By default Firefox doesn't unpack an extension, but merely places the XPI file in the Extensions folder in the Firefox Profile Folder and renames the file to the ID as found in the install.rdf file in the XPI file (ZIP archive). Extensions that need to be unpacked should have an unpack directive (<em:unpack>true</em:unpack>) in its install.rdf file.

Are you trying to install Adblock Plus?

• Adblock Plus: https://addons.mozilla.org/firefox/addon/adblock-plus/

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as to see if that helps in case your security software is causing the problem.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

Now, clicking install for both Adblock Plus and Capture & Print, quickly returns "there was an error installing ___. try again." Faster response. All security disabled.

I tried to install ABP and had no problem, so it looks that the problem is on your side. A while back there have been experiments with signing extensions, but those were stopped and shouldn't cause issues.

Did you try to install the extension in Windows Safe mode or possibly in Firefox Safe mode?

You can also try to save the extension's XPI file to your hard drive (desktop) and install the extension locally via File > Open File.

• http://kb.mozillazine.org/Installing_extensions

Tried safe modes as in previous suggestion. thank you. Out of time for today.