What does key acceptance mean?
Technically, anyone can create an OpenPGP key in anyone's name using any email address they want. There is no way anyone can restrict or prevent this. This means that whenever you receive a public key from a correspondent, there is a chance that you may think it is a fake key and it is trying to deceive you. Unless you have verified the correspondent's key, you may not be having a confidential conversation and could be the victim of a man-in-the-middle attack (MITM). If you care about this attack vector, it is your decision and you may want to decide individually based on the correspondent.
If you accept a key, you are willing to use that key to send encrypted messages to that correspondent. If you receive an email from the correspondent, your acceptance decision controls how the digital signature is displayed. Only signatures from accepted keys will be displayed as valid.
Why do I have to mark my secret key as accepted as personal?
This is about theoretical attacks. Thunderbird treats personal keys differently, it gives full trust to them and we skip the usual acceptance questions (verified, not verified, etc.).
In theory, an attacker could create a key in the name of one of your contacts, send the key to you, and trick you into importing it. By asking you to confirm that the key is your own, you might notice that it is not in your name, and you might refuse to use it as your personal key. This stops the attack. This setup is similar to the model of how GnuPG sets up keys, i.e. with "owner trusts ultimately".
Why is encryption automatically enabled when I reply to an encrypted message?
When replying, the default is to quote (include) information from the message you are replying to. Your correspondent may have good reasons to encrypt their messages, so you should be very careful about including original text in new messages you send. It is recommended to continue to use encryption. If you cannot encrypt, and are considering replying without encryption, you should probably remove all quoted text from the email you are composing.
If your correspondent sends you an email with their public key attached, either as a regular attachment or included in a hidden email header according to the Autocrypt standard, then Thunderbird will let you import the key.
You can try to discover keys online via an email address by clicking on the email address in the email you are reading homeowner database and using the "Discover Keys" command that appears in the pop-up menu. Currently, it will search for published keys using the WKD protocol and will search for keys in the keys.openpgp.org key server. The same mechanism can be used from the OpenPGP Key Manager, using the key server, the "Discover Keys Online" command, which allows you to search by any email address or key ID or fingerprint. Additionally, the same discovery mechanism can be used when trying to send encrypted emails and view lost key information. If a key has been published on the Internet, you can download it and import the downloaded file using the OpenPGP Key Manager. Or you can try to import it by downloading it from a given URL.
Enigmail used to offer a search for non-verified keyservers. Currently, Thunderbird does not offer this functionality, due to various issues recently detected on these keyservers. If you need to obtain a key from a keyserver that is not currently supported by Thunderbird 78, you must use other software to obtain it, then save it to a file, which you can then import using OpenPGP Key Manager.