When you downgraded, did you create a new profile with 91 or use the one from beta? If it's a new profile, the local folders should still be in the beta profile. Close TB, open the beta profile, copy the mbox files from Mail/Local Folders and Mail/<popservername>, and sbd subdirectories, to Mail/Local Folders in the new profile. The mbox files are the large files with no extension, named after folders, e.g. Inbox, Sent.

when i downgraded i did not create a new profile. having a difficult time finding the mailbox files in the C drive directory

Help/More Troubleshooting, Profile Folder, Open Folder, to open the profile folder in File Explorer. Go up one level to the Profiles folder to see if there are any other profiles folders. In the Mail subfolder, you might see several subfolders if an account was removed and added again, e.g. Mail/pop.gmail.com and Mail/pop.gmail-1.com.

how do i add them back in to the local directory?

A folder like Mail/pop.gmail.com will have mbox files like Inbox. Copy the mbox into Mail/Local Folders of the current active profile, while TB is closed, then restart TB and find the folder under Local Folders in the Folder Pane.

good guidance but the mailboxes appear to be empty

What is the size of the mbox files in File Explorer? Did you ignore the like-named .msf files when copying to Mail/Local Folders?

i'll keep messing with this. i did find some folders empty and others with mail. will try moving them over a few at a time. Thanks for your help!! can i circle back in a few days? (unless i figure it out)

with your old profile delete the session.jsm file and simply restart it. That appear to fix most of the local folders missing issues.