When you reply to an email in Thunderbird, it shows the previous message and it prefaces it with a message like "[author name] wrote:" where '[author name]' is the name of the person who wrote the original message. You can actually customize Thunderbird to put any message you want as a preface to the original email.

Get to your "Preferences" panel in Thunderbird and select the "Advanced" tab, and then the "General" tab. On a Mac, it looks like this:

The Preferences | Advanced | General Panel in Thunderbird

Click on the "Config Editor..." button in the lower right. Once you do this, you'll be in the "about:config" window for Thunderbird. In the "Filter" enter mailnews.reply_header to see the preferences will be messing around with. It should look something like this:

Filtered about:config panel in Thunderbird

The easiest change to make is to mailnews.reply_header_type. Here are the allowed values and the changes they make to the reply header:

1 -> "[author name] wrote:"
2 -> "On [date] [author name] wrote:"
3 -> "[authorwrote][separator][ondate][colon]"

Option 1 we've already discussed. In Option 2, it adds the date of the original message in the sentence. The interesting option is number 3. This allows you to fully customize the sentence that precedes the original email. The four sections: [authorwrote], [separator], etc refer to the other preferences that include those names. For example the text for [authorwrote] will be replaced by whatever is entered for the preference mailnews.reply_header_authorwrote.

In the mailnews.reply_header_authorwrote preference, you can use %s to indicate the author name. And in the mailnews.reply_header_ondate preference, you can use %s to indicate the date. Empty values are allowed.

So as an example, consider these settings:

Sample reply header settings in Thunderbird

As it is set up now, a sample reply header would be:

You were blathering on about something on [date]:

As you can see, with the large amount of flexibility, you can configure Thunderbird to precede the original message with just about anything you like.