A reliable, safe and easy-to-use program to efficiently import MBOX to Gmail account is eSoftTools MBOX to Gmail Migration tool. It can import MBOX files to Gmail account with n requirements of Thunderbird or any other application. It support migrating MBOX files of any size to Gmail in a couple of moments without losing anything. In addition, it also offers users to convert MBOX to PST, MSG, EML, EMLX, and several more file formats. Both technical as well as non-technical users can use this tool without facing any difficulty.
More info - Import MBOX to Gmail Account
More info - Import MBOX to Gmail Account