Although rare, I have occasionally seen instances where a folder was marked read-only. I suggest checking that in windows file explorer. That may not be it, but worth a check.

My inbox folder has the "Read Only" box blacked out (as opposed to check mark) which is apparently normal and I don't have the ability to change.

"blacked out' doesn't seem to be normal. If this is an IMAP account, you could delete the folder and its msf file and restart thunderbird, which would repopulate the folder.

It turns out, all of my folders, including ones I have created, show the blacked out square for Attributes>"Read Only (Only applies to files in folder)" The files in the folder are not necessarily read only so it is confusing. If I change this setting in Properties and click "Apply" it will revert back to the blacked out square next time I visit the folder. This is why I think it is a common thing across the Win10 platform. My email account is a pop.Mail account.

I am having the same issue...did you get it resolved?

I resolved my issue by deleting the affected folder (Inbox) and then creating a new Inbox folder in Settings. I was unable to move the messages in the affected Inbox folder to Archives and so 10 months of messages were lost. Going forward I am going to try to save messages to Archives on a more regular basis. I have a POP mail account.

I resolved my issue by deleting the affected folder (Inbox) and then creating a new Inbox folder in Settings.

If you delete the two Inbox files in the profile, under the relevant account when TB isn't open, it will regenerate an Inbox of its own accord. This works for Trash, as well. Personal folders, however, do not regenerate. As david says, if an IMAP account, the folder will be repopulated. Not so if it's a POP3 account.

However, regular back up of your profiles to an external source away from the computer is always advantageous as it means recovery is possible not just if there is a problem with TB.