A NEW version here: Rated 5 out of 5 stars
For anyone considering this plugin, I have "re-created" this abandoned plugin. Here is the official Mozilla Addons link: https://addons.mozilla.org/en-US/firefox/addon/copy-as-plain-text/My plugin works fine in Linux, windows, and on Mac too. As of Nov/24/2012, it supports installation up to v20.0a1.
If you want a version that goes up to v1000, see my post on the plugin's dedicated website here: http://forum.infas.net/viewtopic.php?f=36&t=53 My plugin officially supports Firefox, Thunderbird, and SeaMonkey, however it is under testing for other platforms such as Nightingale, Firefox Mobile (Fennec), etc.