Rated 3 out of 5 stars
Although it doesn't assume to be compatible with Firefox 3.5, it runs very fine with it! But we have to manually modify the "install.rdf" file in the extension folder to change the maxVersion from 3.0.* to 3.5.* to do the job. But sometimes it is best to make the extension to be compatible using another one: Mr Tech Toolkit. One star left.
Other problem: the Flat Style doesn't run fine with Thunderbird and Linux (Xfce 2.6): spaces between menu entries are lost! No problems running TB on Windows (XP SP3). Only a bug, no star left.
Last but not least: CuteMenus - Crystal SVG project doesn't seem to be still alive, its website is seems to be lost into the web jungle. Second star left.
Wait & see for further updates (if there will be some)...