Anyone on the Internet can send a specific email to you at any time, and there's a good chance that you will open it. If there's a critical security bug in Thunderbird, the sender of the email might be able to take over your entire computer, including all files on it, reading and modifying all your private photos, confidential business documents, databases, stealing passwords for other systems that you enter on the keyboard, and posing as you on the Internet, and using your access rights to attack other computers in your company.
That might seem unlikely, but that sort of thing unfortunately does happen rather often, including to very reputable institutions. Most people ignore the risk, until it happens to themselves. For the victim, this is a nightmare.
Unfortunately, such critical security bugs in software are occurring rather often (including in security software, ironically), and are fixed on a regular basis. They often do no harm, provided that they are fixed quickly and you install the updates regularly and swiftly. Each day without the fixes increases your risk. This is particularly true given that the exploits often are made public after the fixed software is released, so not installing updates puts you at great danger.
The fast nature of the software code changes also mean that the fixes cannot be applied on older Thunderbird versions. In general, only the current stable release is fixed, and older versions are left vulnerable by Mozilla. This is why it is highly dangerous to use unsupported Firefox and Thunderbird versions.
For that reason, we do not support ExQuilla on old Thunderbird versions in any way. It would put our users at great danger, and we will not do that.