Right-click the Add to Firefox button and use Save Link As...

To install it into Firefox - drag the .jar file into the Appearance tab, but you'll need to "bump" the MaxVersion in the install.rdf file first.

Be careful with forcing compatibility on themes as this can easily break Firefox or cause problems with icons like the Site Identity Button that has recently changed.
It is better to wait for an official update in case of themes.

• https://support.mozilla.org/kb/Site+Identity+Button
• http://blog.mozilla.org/ux/2012/06/site-identity-ui-updates/

I think the problem is that the theme is good to version 14 and firefox is at 14.0.1

How do I bump the max version ?

see if this helps : https://support.mozilla.org/en-US/questions/932988

thank you