From MozillaZine Knowledge Base

Thunderbird 3 has the ability to automatically quote only the selected text in a message you are replying to.

The following information applies to older versions of Thunderbird...

Thunderbird does not have the built-in ability to automatically quote only the selected text in a message you are replying to. You can set up Thunderbird to quote the entire message or not quote it at all (go to "Tools -> Account Settings -> [account name] Composition & Addressing" to change this setting). To selectively quote, you can either:

• Click the Reply button and then manually delete the parts you don't want when the Compose window opens.
• First set up Thunderbird to not quote by default. Then when replying: manually select the part you want to quote, copy it ("Ctrl+C" or right-click and select "Copy"), click the Reply button, and then in the Compose window that opens right-click and select "Paste as Quotation".

Additionally, there are a few Thunderbird extensions that support selective quoting:

• ResetQuoteHeader extension
• QuickQuote
• QuickReply

Be sure to read about each extension and how it works, as they seem to support selective quoting in different ways.

Other information

• Bug 23394: view the status of the enhancement request or vote for it (but please don't add unnecessary comments)