Links clicked in Thunderbird open a new Seamonkey window instead of opening a new tab in currently open Seamonkey

This issue has been causing me major inconvenience. I got hacked awhile ago. I mean, they got through my router. They copied all my files. This event has made me a bit paranoid of hackers. I had to change all my passwords, to new passwords that don't reflect me at all. One of these was my gmail password. Thunderbird used to ask me for my password, but new Thunderbird doesn't. It simply stores my password when I set up the account. This has been a problem in and of itself, because every time I reset my gmail password, I have to go to my terminal and rm ~/.thunderbird. Just removing the account doesn't seem to work. It won't accept my new password UNLESS it believes itself to be a brand new installation. I don't know how this problem wasn't picked up in the beta testing phase of development, but AFAIK it still exists. This is not the problem I wish to solve in this thread.

The problem I wish to solve in this thread is the problem of clicking on links in emails. When I click on a link, it SHOULD open in a new tab in my current Seamonkey window. It does not. It opens a new Seamonkey window with Done at the bottom. It does not even try to open the link. The workaround I've found is to right-click the link and select Copy Link Location and then open a new tab in my current Seamonkey window and paste&go. Can anyone advise me on how to get back the behaviour I expect from Thunderbird, and that I described above? Thank you in advance for any advice you can offer on solving this issue. If it matters, I am running mozilla-thunderbird 68.5.0-x86-64-1_slack14.2.