You have posted an image that generally indicates you have an antivirus product interfering in mail by scanning encrypted communications, or you are using encrypted communication perhaps not specifically supported by your provider. As your image is blurry I have no idea what server it is, but generally these are best investigated by viewing the certificate to determine the server they are issued for and who issuesd the certificate.

Your text speaks to not specifically requesting mail. I have no idea what default settings you have changed, but if your mail is on an IMAP account and Thunderbird is online you are unlikely to be able to stop mail being downloaded, especially if you have received mail from the server once in the session. Spam is really only normal mail with an arbitrary label attached, generally without any real definition. I have seen everything from they are not in my address book to malware described as spam by different folk over the years. Unfortunately SPAM as such defies definition.

Hello Matt, It appears that an image was somehow posted with my description of the problem. That was not posted by me and if it somehow was attached to my question I do not know how that happened. Next, in your answer, I do not understand what you are telling me. I am simply a user of Firefox for my emails. I do not use any encrypted communications. My server is Centurylink which is the Phone Company in this area. To download any email from the server I select one of my email accounts and click on the "Get Messages" button and the server sends my messages for that email ID. It sends the email I request and only that email. But somehow the junk stuff gets downloaded into my 3rd email address but never does any email ever get down loaded into my main ID or my 2nd ID unless I select that email specificly and then click the "Get Messages" button. But lately somehow all this junk is downloaded without ever using the 3rd email and asking for a download. All 3 email accounts were created under Firefox several years ago and set to the same controls. Firefox has changed so much in the last few years I do not even recognize the setup for a new email account or even the display of the ones I have when I select the name of one of my accounts which are all the same age. I have no idea how to create a new account under Firefox any more. As I have been forced to replace my computers each time a new Windows has come out I have been able to transfer my date files over to the new computer. This includes Firefox and my email stuff.

Hello Matt, It appears that an image was somehow posted with my description of the problem. That was not posted by me and if it somehow was attached to my question I do not know how that happened. Next, in your answer, I do not understand what you are telling me. I am simply a user of Firefox for my emails. I do not use any encrypted communications. My server is Centurylink which is the Phone Company in this area. To download any email from the server I select one of my email accounts and click on the "Get Messages" button and the server sends my messages for that email ID. It sends the email I request and only that email. But somehow the junk stuff gets downloaded into my 3rd email address but never does any email ever get down loaded into my main ID or my 2nd ID unless I select that email specificly and then click the "Get Messages" button. But lately somehow all this junk is downloaded without ever using the 3rd email and asking for a download. All 3 email accounts were created under Firefox several years ago and set to the same controls. Firefox has changed so much in the last few years I do not even recognize the setup for a new email account or even the display of the ones I have when I select the name of one of my accounts which are all the same age. I have no idea how to create a new account under Firefox any more. As I have been forced to replace my computers each time a new Windows has come out I have been able to transfer my date files over to the new computer. This includes Firefox and my email stuff.