Sending HTML email using Mozilla Thunderbird - plain text only option

I appreciate you laying out the details of your concern and adding a screenshot, @DKRat. 

I’d be delighted to provide some alternatives, so you’ll be able to deselect the Use "Plain Text" format for e-mailed invoices option in QuickBooks Desktop.  

Since the option is greyed out when selecting the Mozilla Thunderbird option, you'll want to use the Web Mail option instead to unmark the Use "Plain Text" format for e-mailed invoices box. Once done, select the Mozilla Thunderbird option again, and you can now utilize it without using the plain text email feature.

I’m happy to provide some screenshots for additional reference: 

For more insights about setting up an email service in QBDT, feel free to open this article here: Connect your email to QuickBooks Desktop. 

Moreover, you can pull up this handy guide just in case you’ll want to send multiple invoices at once: Email sales forms, invoices, and statements in QuickBooks Desktop.

Please feel free to keep me posted on your progress when setting up this feature in QBDT. I'm determined to help you succeed. Keep safe always! 

Thank you for trying to help @Carneil_C 

I tried the steps you mentioned but they did not work. I was able to see the unchecked box from the Web Mail option, but if I switched back to Thunderbird it would not work.

Another coworker called and was informed that the only version of Thunderbird that does work is v60. So we may not be able to use Thunderbird in this way.

Thank you for trying to help here, @Carneil_C 

Unfortunately, that did not work. I was able to switch to Web Mail and the check box is unchecked, but when I switch back to use Thunderbird the option is grayed out and will not switch.

Another coworker called Intuit and found out that the last version of Thunderbird that worked was v60. If anyone wants to use this with Thunderbird from 2018, then that seems to be the only way.