Are you trying to upload mail from Local Folders to a folder on the gmail server? That's not as straightforward as it appears:

https://support.mozilla.org/en-US/questions/1331879

You might have better luck by setting the new account to collect mail from the other account, through the gmail website.

Also, an add-on isn't needed to import mbox files to TB Local Folders, although for TB 115 there is one.

https://support.mozilla.org/en-US/questions/1417914

Hi sfhowes,

that is exactly what i am trying to do! the gmail website works well for forwarding new mail, but is atrocious by design to import old mail,

I was able to add the Copyfolder add-on to thunderbird, but right clicking on the the mail that i wanted to copy didn't pull up any option to use the extension. I then proceeded to download thunderbird version 78, had to relink to the gmail profile, and now all of those emails that were stored to my local folder are gone. I see multiple profiles in my user\library\thunderbird folder, which i guess is where the emails are stored if i'm using Pop?

I attempted to run Profile Manager by entering "/Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin -ProfileManager? in terminal but i'm getting an 'Unhandled error during faulting  Error Domain=NScocoaErrorDomain Code=134092"

in the following thread you mentioned- about the add-on not being required to import mbox files. for version 115....however they are saying that the files are .eml: /Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin -ProfileManager

oh forgot to mention the reason I can't do all this through gmail it's transferring from a g-suite account to gmail... .mbox becomes the only option

Open gmail in a browser and then look at Settings, Accounts & Import, and try the Import mail & contacts or Check mail from other accounts to collect mail from the other gmail account.

It's probably easier to manage profiles from Help/Troubleshooting Info, about:profiles.

POP mail is in the Mail/<popservername> subfolder of the profile folder. An mbox file can be imported by copying it into the Mail/Local Folders subfolder, while TB is closed. To import eml files, launch TB and create a subfolder of Local Folders, then add eml files from Finder (or Win File Explorer) by drag and drop to the subfolder. Both mbox and eml files can be imported/exported with the IET NG add-on, but it's not necessary for simple operations.

that method doesn't work from gsuite to gmail.

I can see the transferred mail in my Thunderbird folder - mail\pop.gmail.com\inbox.

Is there another step to be able to view in gmail? I have POP3 enabled in that account

The problem was solved by removing the Pop server and using IMAP instead. Thank you for your add-on suggestions- they were the key. However it aborted leaving about 10K emails left. If I attempt to run the copy folders add-0n--> the one that was linked in the thread that you originally sent (78-copyfolder.xpi -- only works with TB ESR 78.*, will this copy duplicates? Is there a way to only copy the emails that got left out?

now the copy folder option when I right click has dissappeared. I reinstalled in the add-on to no avail