Review for Attachment Sizes by theonlyandy
Rated 4 out of 5 stars
It's really great to see the size of attachments before sending an email!
Also great is the display of KiB instead of KB - that helps that people get aware of the difference: KB are 1000 B, KiB are 1024 B.
But I'm really missing the display of attachments with messages on IMAP accounts.
To create your own collections, you must have a Mozilla Add-ons account.