With three easy steps you can free a lot of space in your inbox with the help of Mozilla Thunderbird.
1. Click on the little icon in the upper right to „Select columns to display“ and show „Size“ column.
2. Click on the new „Size“ column to order by size
3. Start with your biggest email, go to the attachments and select „Detach all…“ and save them to your harddisk.
Repeat this process for your biggest emails and quickly you will have a lot of free space in your inbox again.