Request cannot be edited? -I've found out that due to limitations in newer versions of Firefox it is not possible to edit data, so the developers are not to blame for this.

You can't tamper (change or add new) data with this. You can capture request and visually check what is inside.

why can not edit request parameter,modify the post data?

==Cons:==
As commented by other users before:
1. This extension cannot edit/tamper the request, that is **NOT** like the old "Tamper Data". The name and description are both misleading. Not sure if such kind of tampering is technically infeasible or just the author need more time to implement it.
2. The UI is awful and default window size when starting tamper exceeds my screen.

==Pros:==
Anyhow, this extension still can list all the parameters of the HTTP header, which provides a easy way to analyze your HTTP/HTTPS requests, especially for HTTPS, comparing to other approaches. That's why I give one more star.