Well if you are looking for a way to download email from gmail, and then open it offline, here is the best way to do it.
1. Click https://www.google.com/settings/takeout
2. Click Create an archive.
3. Select the product, like gmail, calendar etc. You can specifically select a label of your Gmail too.
4. Create Archive.
5. You will get an email once archive is over, download it. That's all.
Opening the downloaded mails using Thunderbird.
1. Launch Thunderbird.
2. Left side you will find local folder in Thunderbird.
3. Click local folder and check properties to find the physical location of the folder.
4. Extract the archive file of your gmail, you will get .mbox for mails.
5. Copy that file to the local folder of Thunderbird.
6. Restart Thunderbird and you will find your mail under local folder.