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.