If the original gmail files are .mbox files, copy them into the Mail/Local Folders location of the profile folder, with TB closed, remove the .mbox extension, then restart TB and find the folders under Local Folders in the Folder Pane.

Thank you so much. One more quick (?) question, tangentially related: what is the safe way to change the local folder location? Thank you in advance

Copy Mail/Local Folders to the desired location, with TB closed, then Account Settings, select Local Folders in the left pane, Browse... to the new location of Local Directory.

http://kb.mozillazine.org/Moving_your_mail_storage_location_-_Thunderbird

See also https://addons.thunderbird.net/en-us/thunderbird/addon/localfolder/

I recently completed a new download of Gmail in order to archive the messages. I have them in Thunderbird and I can search for some messages but I only get snippets and I want to read the whole message. When I try to access the message Thunderbird tells me "no message"

Was this a Takeout download of .mbox files or download in TB? Are you downloading headers only or the full body (Account Settings/Sync. & Storage, Message Sync.)? Are you downloading a large number of messages within the daily gmail limits?

It was a takeout download from Gmail

One more thought: When I am searching, I am typing in a string of words or a phrase. This returns the "snippets" as I mentioned along with information regarding attachments that were in the original message. I assume this is creating a filter. There appears a header that seems to be linked but when I click on it I just get another tab open with "no messages". Also, I can't select or copy any text within the thumbnail

I can't visualize what you mean by a snippet or what kind of search you're doing (there are 3 kinds). Post a screenshot that indicates the issue. Also, check the account in gmail webmail and see if the messages are visible. The Takeout download just includes the content as it appears in webmail.

Here are three screenshots. One shows folder contents. One shows my filtered search list. Each thumbnail (or "snippet" as I call em) shows some limited text of the messages, highlighting the filter criteria. The subject of the original message is in bold, appears to be a hot link. When clicking on it, hoping to read the entire text, it opens the blank tab with "no message" showing. That is shown in third screenshot. One more thing: I can't check the messages in Gmail web as they have been deleted (one of the reasons I did the takeout to store them in Thunderbird)

Thanks for your reply and your patience

You're using Global Search, which might need a database rebuild after you imported the mbox files:

https://support.mozilla.org/en-US/kb/rebuilding-global-database

But since you're searching a specific folder, you're more likely to get better results with Classic Search: select Archived.mbox in the folder pane, then Ctrl+Shift+F, and enter the search criteria. Even faster, enter text in the Quick Filter box (Ctrl+Shift+K), then click the Sender, Recipients, Subject or Body buttons.