Here's how to maintain an account in Thunderbird, but "disable" it so that it does not fetch messages when you click "Get Mail" (or when mail is retrieved at startup or periodically):

Tools >> Options >> Advanced >> Config Editor...

In the Filter bar, enter "server.server". Thunderbird apparently stores properties for each account under config items that have names like "server.server1", "server.server2", etc. So filtering on "server.server" will show you ALL of your accounts.

In the list, look for an item named something similar to this:
mail.server.serverX.name
...where X above is actually a number (1,2,3,...), depending on how many accounts you have.
...now browse through the list and look at the values of the items named "mail.server.serverX.name"...find the value name that corresponds to the email account you want to disable.
For example, if you discover that "mail.server.server6.name" has the email address for the account you want to disable, then...
...in the list, look for (or filter) "mail.server.server6.defer_get_new_mail" ... set this to false.

Thunderbird will no longer check that account for email.

This is obscure...but it looks like it works.