It appears to be corrupt, according to Firefox.

[Spirogira]

I'´m getting an error stating Firefox can't install the .xpi as it appears to be corrupt.

[retorquere]

Right-click and save the extension instead of left-click it to download. With left-click, Firefox thinks it's a Firefox extension and tries to install it in Firefox instead of zotero.

[Spirogira]

Oh, ouch, this was a dumb one! Thank you for the attention.

[retorquere]

It hits every Firefox user the first time. It's a stupid error message.