Rated 3 out of 5 stars
Correction to my post of August 14, 2012: It does work in SeaMonkey Messenger 2.11. The trick is to open a mail composing window, right-click on the Mail Toolbar, select Customize... from the context menu, then drag the Highlighter icon from the palette to where you want it on the toolbar. Unfortunately, the current incarnation of SeaMonkey doesn't allow one to drag the icon to the Formatting Toolbar (called "Formatting Bar" in Thunderbird), where the icon is automatically installed in Thunderbird, but at least it works. The instructions for this add-on need improvement, both here and at the developer's site. In the alternative, I'd like to see the add-on modified so that it installs in SeaMonkey Messenger as easily as it does in Thunderbird. It is frustrating to use this application when the installation is nebulous and experimentation is needed to get it to work. That's why gave it only three stars.
I found the pink highlighter a bit too dark, trending more toward magenta, so I tweaked the code in the overlay.js file, changing it from 'fdf' to 'faf' (hex colour values), and re-zipped it to create my own customised version of Highlighter xpi file, which I then installed in SeaMonkey.
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.
Works on SeaMonkey Rated 4 out of 5 stars
Highligher 0.6.5 works on SeaMonkey 2.8 after I edited the install.rdf file in the XPI archive to change maxVersion from 2.1a to 2.8, then re-zipped the files into a new archive and changed its extension from .zip to .xpi. The Highlighter widget appears at the extreme right of the Formatting Toolbar, and one must have a screen width of at least 920 pixels to be able to access it. It would be nice if there were an option to move it to the Mail Toolbar, which has fewer items and would allow the window to be as small as 640x480 (standard VGA). In the alternative, the highlighter function could be added to the Format menu.