Try downloading from here if you have not already tried.

Please ensure that McAfee is not interfering with the installation process.

I shut my anti-virus off and tried to install for the link you included. Same results.

Ensure that there is not a user.js file located in your Firefox profile folder or the program directory.

If there is, please delete it.

You get this error if you install an extension that needs to be unpacked in order to work properly, but this isn't happening. The default is to copy the file to the extensions folder and rename the file part to the ID of the extension.

• https://developer.mozilla.org/En/Updating_extensions_for_Firefox_4.0#XPI_unpacking

You can try to set the extensions.alwaysUnpack pref(s) on the about:config page to true.

It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the extensions.* files (e.g. extensions.sqlite, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled or not compatible extensions in "Firefox/Tools > Add-ons > Extensions" then click the Tools button at the left end side of the Search Bar to check if there is a compatibility update available.

If this hasn't helped then also delete the addons.sqlite file if present.