Thunderbird can be downloaded from https://thunderbird.net/ and then installed.

Hmmm. I can't recall if this is the link I used the first time. Will my email and other settings remain in place if I install it again? Also, I had two accounts but after disabling add-ons, the other one is missing?????? Thanks!

This implies that you do have Thunderbird installed, yet the screenshots don't reflect it. Can you post a screenshot of Thunderbird screen?

Another member says it looks like it's installed in a browser and suggested I go to this site and download and install.

True, you can download thunderbird from thunderbird.net, but you stated you have email accounts set up and you referenced using addons, which implies that you also have thunderbird installed. I am wanting to assist you, but your responses are confusing.

OK, if I restore add-ons, what do I do next to get this right?

If your addons caused this problem in the first place, I wouldn't be looking to reinstall them. If not now, you'll likely have trouble in the future.