The challenge you face is that the email content must be fully formed prior to Thunderbird sending it. That reformatting would need to be done by an external process. An option might be to create multiple identities, each with a different quote, and pick one as desired.

David thanks for the follow-up. Can you share more details about what you're suggesting?

Also, in addition to what you've suggested, area there any other possible options?

Thanks.

Jeffrey

To my knowledge, all options require work, some much more than others. The easiest that I know of is to create multiple identities, each with different quotes. To do that:

1. click account settings and highlight the desired account
2. on account pane, click 'manage identities'
3. add an alternate identity, changing name slightly to ensure uniqueness, keep same email address but a different sig

Now, when selecting 'new message, the FROM drop-dowm will alllow choice of from address.

Another option, possibly easier, is to use the 'signature switch' addon.