Review for Edit Email Subject (EES) by rumplestilts
Rated 5 out of 5 stars
Absolutely essential add-on. However, one minor suggestion:
When triggering the dialog, it would be helpful to have the entire subject line already selected so one may simply begin typing to replace it (rather than first pressing Ctrl-A/Cmd-A to select all the text). Admittedly, a minor quibble.
To create your own collections, you must have a Mozilla Add-ons account.