If you have the Folder Pane Header visible, right-click the cloud icon at the left end to choose a specific account.

The option to include servers when getting new mail only applies to POP accounts. Do you have POP or IMAP?

POP on all accounts. I had hidden the folder pane header so I brought it back and see the option you mention, thanks for that. But while it works, it's in an extremely awkward location for right-handed people. I'd much rather have it on the Toolbar, where I can put my most used buttons farthest right.

Which brings up another issue - while I like having less vertical space wasted with stacked rows of menu, toolbar, etc., why does everything that used to be on the menu bar have to be fixed at the right end of the toolbar?? The settings and window control icons total FOUR items that I rarely use, displacing my most used icons out of the most ergonomically friendly spot for a right-hander, and unlike everything else on the toolbar, are not movable. I started feeling the increased repetitive strain this causes within 24 hours of the update :(

If you right-click the Unified bar, Customize..., a Get Messages button can be dragged onto the bar. Clicking that button fetches mail for all accounts, as does Shift+F5. Clicking the cloud icon on the Header bar gets messages for the current account, like F5, or right-clicking allows choosing the account.

There are various userChrome options to hide, move or resize the toolbars, and e.g. move the app menu icon to the left side. The customization is maybe not as easy as many would like, but it is possible.

I repeat, they took a very convenient option, moved it to a very inconvenient place, and hid it. Not my idea of an "upgrade."

The other part of my question, how to EXCLUDE a specific account from the all-messages download, has not been addressed. There used to be an option in the account settings for that, which has now disappeared. With the expanding number of people now managing multiple email addresses, some of which may be work addresses that they do not wish to monitor in their off-work hours, this seems like a really inappropriate time to remove that option.

In my case, I have an old account that receives almost entirely spam, but which I wish to keep, as it my only means of connection with certain correspondents. Since it receives multiple spam messages every day, I REALLY don't want those downloading in real time, which renders it impossible to tell from the notification bubble how many real messages I have. For years, it has been my practice to manually download them once a month, and clean out the spam messages. The "include this server when getting new mail" checkbox used to be an option for accounts with their own inbox, but apparently now it is only available for accounts using the global inbox (which all of my other accounts do). I don't want to send the mostly spam messages from this account to my global inbox to get intermixed with all my other mail.

Every time there is a major update of Thunderbird, options I use every day are suddenly deleted, disrupting my workflow, sometimes in a major way, and leaving me with the unavoidable impression that Mozilla is seriously out of touch with real-world usage of Thunderbird, and not very interested in correcting that, or even seeing it as a problem.