Rated 3 out of 5 stars
Stopped working in SeaMonkey 2.9 and later, even with the maxVersion value tweaked in install.rdf. SeaMonkey 2.11 doesn't complain or automatically disable the plugin, but the highlighter no longer appears in the formatting toolbar when composing a message. It was useful while I was still using SeaMonkey 2.8 and I hope to see this tool updated so it works with later versions. Without it, one must resort to tedious HTML editing to highlight text, using <span> tags with background-color style information.
Still works in Thunderbird 14, but that's not the mail client of choice for me.