Same as Ctrl+Shift+V Rated 3 out of 5 stars
If you type Ctrl+Shift+V in Thunderbird (or most other applications) you have this behavior already. No need for this add-on if you know this keyboard shortcut.
Or click Edit > Paste Without Formatting. Also an option that most applications have, including Thunderbird.
Still, for applications that don't support this, there may still be use for _copy_ as plain text, as opposed to _paste_. For which I give it three stars.
There -is- a difference
I appreciate your feedback comments, however there is indeed a difference between CPT-EVE and Ctrl+Shift+V.
The default options for CPT-EVE will render what appears to be identical results to the keyboard shortcut to which you refer. However, CPT-EVE can be customized with options that will give you a more organized paste than the keyboard shortcut.
I have made an .odt document that demonstrates those differences, it can be viewed and/or downloaded here: http://bit.ly/xRPvCa