In the usenet you find more or less as a standard for quotations the quoteinitial ">". This is used to quote text of previous postings in the same thread.
To make it easy to distinguish, when you use a quotation whose orign is may be a webpage, usually the writer uses another quoteinitial: "|"
The newsreader "40tude Dialog" e. g. offers two different commands for "pasting as quotation".
One standard way with ">" and a second command which pastes the text of the clipboard with the quoteinitial "|".
I think such an option is very useful and I like to suggest to add this option to thunderbird.
Dieses Anliegen in deutscher Sprache (this feature-request in german language):
http://borumat.de/thunderbird-email-tip ... rnes-zitat