https://support.mozilla.org/kb/cannot-send-messages

I have already checked out everything in the suggested link. Everything is the same as it always was prior to this issue. The only thing that I can see that changed are updates.

Start Windows in safe mode with networking (Shift + Restart) and see if you still cannot send.

I can send when I run Thunderbird as administrator. I just cannot send when I open Thunderbird normally. If there were problems with any of my account settings I would be unable to send messages period. I cannot pinpoint the exact day or time this issue developed because I don't send many emails. All I do know is that there were program updates between the time Thunderbird worked normally and when I discovered this issue.

I did boot into safe mode with networking and was able to send an email by opening Thunderbird normally, but when I booted back to normal Windows mode, I got the same results as previously noted. The only way I can send in normal Windows mode is to run Thunderbird as administrator.

If it worked in Windows safe mode, that indicates that a program that is disabled in safe mode is causing the problem in normal mode. The obvious candidate is Norton Internet Security, so I suggest you add the TB profile folder (Help/Troubleshooting Information, click Open Folder) as an exception in NIS or disable the email scanning component. You may also have to exclude the TB program files folder.

First of all, I have been running NIS, and for that matter Malwarebytes, long before this issue arose. But for curiosity sake, I attempted to send a message with both those programs disabled. Still not able to send. If there was a program that is disabled in safe mode that is causing the problem, would not that same program prevent me from sending, when I run TB as administrator in normal mode?

You have shown that it works in Windows safe mode, so I suggest you compare the running processes, via Task Manager, in safe and normal modes, and try to figure out which program causes the problem in normal mode. NIS is a likely source, but not necessarily the only one, and besides it's not sufficient to disable it in normal mode to guarantee that all of its residual effects are removed.

I will certainly try the last suggestion, but it still comes down to the fact that it works in normal mode when I run TB as administrator. Updates to TB will not install unless I'm running as administrator. I am having similar issues with Firefox. Unless I am running Firefox as administrator, Firefox will not update. Downloads for programs from other websites fail unless I am running FF as administrator. I cannot even install addons unless I'm running FF as administrator. I receive an error: addons.mozilla.org. (null) could not be installed because firefox cannot modify the needed file. All of these issues unexpectedly began popping up within the last few months, and I feel that there is a common link somewhere. I have no issues with other programs I have installed on my computer, only Mozilla related programs.

Maybe as a stopgap measure you should check the 'Run this program as an administrator' box as shown in the picture.

Thanks sfhowes. Doesn't answer the question of what caused all of these issues to begin with, but does make my life a little easier. For security concerns though, I will not be doing the same for Firefox. Will only run that program as administrator when needed for updates, downloads, etc.

My guess is you are using NIS at an increased security level. When you get it out of the box it has a god awful settings, but increasing the levels of security in the firewall or any other part of NIS is almost guaranteed to cause problems with other programs.

There are many folk that love the Norton products. They lost be about the time Symantec bought the brand. Perter Norton made brilliant tools and his anti virus was a wonder, a corporation bought it and in my opinion it now includes the kitchen sink and It slows you computer significantly. (some 10%)

The same story goes for McAfee, it was good. Then John McAfee sold the company. Look at it now. Intel are not doing it any favors

As an example. Do you understand all this stuff. https://support.norton.com/sp/en/au/home/current/solutions/v9802264_ns_retail_en_us I do not understand it. It does have very serious impacts on what you can do on your device that most of us determine by trial and error. But I do not understand what they are saying.

Matt, I checked my settings in NIS and all appear normal. But I am happy to report that I stumbled upon, what appears to be the solution, though I don't understand why it worked. As I stated in my 1st post, I am running Windows 10. I actually did the upgrade from Windows 7. So, now the fix. I ran the Program Compatibility Troubleshooter and it told me that the TB program was not compatible with Windows 10. I ran the fix and now I am compatible with Windows 8. I undid sfhowes suggestion, opened TB normally, and was able to send myself an email, which I received a short time later. I then ran the troubleshooter for FF, and it told me the same thing. I ran the fix and now FF is compatible with Windows 8. I will not know whether it solved all the issues I had with FF until I need to download something, but I'm hopeful. Would appreciate feedback on the fix, and thanks to all.