Just how are you trying to do this?

I am trying to copy them by selecting them and dropping them in an inbox folder.

Selecting what... the mbox files or the emails they contain shown in the Thunderbird user interface. Really make yourself clear. I tire of asking for simple information.

How many emails at a time. My understanding is more than about 500 will probably fail for one reason or another.

I am selecting the mbox file and attempting to copy it to an inbox folder.

Isaiah6200 said

I am selecting the mbox file and attempting to copy it to an inbox folder.

I am not sire why you think that would be an effective way to get email either into Thunderbird or onto a gmail server. Try importing the contents of the mbox file to a local folder before you try to do anything else with the emails.

https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/