Color of quoted text in email editor

See this question at Mozillazine. You can do this by changing your userContent.css file.

The code you'd need to add is

span[_moz_quote=true] {
    color: pink ! important;
}

changing 'pink' to whatever color you prefer.

On Windows XP, you'd find the file in C:\Documents and Settings\user\Application Data\Thunderbird\Profiles\xxxxxxxx.default\chrome (the chrome sub-folder does not exist by default); see developer.m.o for location on other OSes.