Please provide details on what you are attempting. The desktop thunderbird imports mbox files from where? A different email account? You're wanting to keep the messages on the phone, but not in the gmail account? Maybe a walkthru of the steps will help. Thank you.

I thought I had explained. You're not an AI bot, are you? It is a commonly recommended procedure for transferring emails in bulk between Gmail accounts to use Takeout to download a huge mbox (or separate labels), copy the mbox into .thunderbird/<profile>/Mail/Local Folders, then use Thunderbird to move them into the new account by IMAP. The only import available in Gmail uses POP3 and is highly unreliable and unsupported. I used this procedure to move out of my Google Workspace domain in 2021 when it became no longer free and again in 2023 when free account email storage became included in the free 15G allowance. I divided my historical emails 1988-2016 in one account and 2017-2025 in another. This year I had to move the boundary from 2015 to 2017 and I have only Android and Chromebook available. Thunderbird is failing in Crostini, which I have reported as a bug.

No, no bot. From your post, i infer you use chromebook as the desktop to upload messages to gmail. If you're uploading to an email account, I don't see the need for a separate storage (Local Folders) on the phone. Thunderbird needs an account to move messages to, not a fixed location on a phone.

To repeat, the one feature which keeps me and I'm sure many others using Thunderbird is its handling of mbox files. This is missing from the Android app, which seems to be just a re-branding of Kmail, which IMHO should not be allowed by the Mozilla Foundation, who should have adopted Kmail with its name. The feature could easily be added, even improved by putting Local Folders in, say, storage/emulated/0/Thunderbird.

You also suggest I use the Chromebook, but I already said that desktop Thunderbird no longer works in its Crostini linux container (bug 1954274) though I do have a slow workaround using Termux, Tigervnc and xfce4. It would be so much easier with Android Thunderbird on the Chromebook.

You will want to request this at https://connect.mozilla.org/t5/ideas/idb-p/ideas/label-name/thunderbird%20android