If the default account isn't the one you want to send the message from, you can pick the desired account from the From: field drop-down menu in a Write window.

I don't think you read the explanation of the issue carefully. You answered the question: How do you change the sender? That's easy. The question underlying the issue is: How do you change the default server that comes up when you click on a mailto link in a website? But I have the answer: In Account Settings, select Account Actions, then select Set as Default. The issue I raised is not about how to set the sender, but about help. My question is why did it take so long to find the answer? Am I navigating correctly? Was there a better way to ask the the question? Is there a searchable user manual that might have provided the answer more quickly? If not, why not?

I am confused on your statement on why it took so long to find the answer. You posted it here and @christ1 gave a valid response, clear and lucid. So, whatever your concern is, it's not coming through. I viewed your earlier posts and they didn't address this.

Please read my original post and my reply to @christ1 carefully. I raised an issue about how much time it took me find an answer to a question by searching through Thunderbird help, including this forum. As an example, I used the time it took me to find an answer to the question of setting the default sender. @christ1 did not answer the question in the example, but answered another question. I didn't need an answer to either question. I was raising the issue of how long it took to find an answer to what turned out to be a fairly straightforward question: How do you change the default FROM address that comes up when you click on a mailto link on a website? In my reply to @christ1, I pointed out that I already had the answer to that question. I had given it my original post. So there was no need to attempt to answer it (and it was irrelevant to provide an answer to a different question). I am not objecting to how long it took @christ1 to reply. I am complaining about how long it took me to find the answer on my own. The title of my post begins "Help not helpful." Users of this forum should search the forum before making a post in case the question is already answered. That's what I did. I searched for changing the default sender. After reading many irrelevant queries and answers, I finally found the answer buried in a long thread. So my question again is: Why did take so long? How could I have better asked the question? Is there a user manual? If not, why not? As I noted in my original post, the meaning of "default sender" is ambiguous. The first question someone attempting to reply should ask is: What do you mean by default sender: 1) the "mailto sender" 2) the SMTP for an account or 3) the default identity for an account? The question @chist1 answered is none of these, because the answer addresses changing the sender while sending the message, not changing the default.

This is a forum, not a help file. It is not uncommon for questions to be posted that have nothing to do with the real problem, often causing the answer to be several levels deep as we work with the individual in validating the true issue. That's a value we contribute. I share your wish that answers would be easy to find by the individuals doing a search. In fact, I wish more users would browse first, as I see questions on several topics repeatedly posted. Generally, when a person posts a clear question, they receive an answer within a fair period of time, but few questions are of that ilk. A challenge is that there are multiple resources. I just was at the knowledge base: support.mozilla.org, and entered 'how to set default email address' and the solution appeared immediately, but that does not mean all questions are immediately answered. Yes, it does sometimes matter how the question is asked. So, to me, there is no answer to your question. This forum is a living entity, as is the knowledge base. In general, if you are unsure how to do something, come here and post your questions. That usually works. Thanks for sharing your view.

I followed the link to support.mozilla.org and entered 'how to set default email address'. Yet, the solution I was looking for was nowhere to be found. Please tell me what solution you found. Was it " In Account Settings, select Account Actions, then select Set as Default"? If so, under what question did you find it?

You write, "This is a forum, not a help file." But I wasn't asking for help, I was posing a problem for discussion, which two "top contributors" misread. I am trying to help make Thunderbird easier to use by pointing a way in which it is not easy.

Nevertheless, saying this isn't a help file is disingenuous. The website support.mozilla.org is where you end up when you click "help" in Thunderbird and when you key something there you end up in this forum. That is, this forum is where Thunderbird sends you for help. If there isn't a user manual, perhaps Thunderbird should have one. Wouldn't that be better than searching through random answers to random questions? That wouldn't answer every question, but it would save you and the many, many people who depend on Thunderbird hours of search time. Perhaps, in the absence of such a manual, to help others, I should repost the question that prompted this thread and then answer it myself.