Have you tried downloading the modified version that some guy put up? Quoted from the add-on page at Mozilla

by Sudowudo on August 16, 2011

Updated to work with Firefox 6, I set the max version to v9 so should be good for a few more versions.
http://www.mediafire.com/?ld1s51fkzte3d83

This is such a popular extension I cant understand why it hasn't been kept officially supported.

So unless it fiddles with new internal code, it now says it'll work with higher versions in the installation info