Some months ago I wrote this rating, giving only 1 star. Since then Firefox evolved and the most annoying bug vanished:

[stroke]When I add a OAuth key the first time, I have to remove the modal overlay manually using developer tools.[/stroke]

Most bugs still persist:

Editing behaviour of request body text field is unexpected:
You click in the field and the box grows in size, reflowing text. Then it annoys you further, by selecting the whole text, instead of letting the cursor just be.

If you type in the URL field, it suggests previous URLs (coming from the browser, not addon). Good! When you navigate with the arrow keys to the desired item and press Enter, the request is directly sent. Bad!

Animations are really slow and distracting.

In the header dropdown, you don't see all of the content, e.g. there is an entry "Content-Type: appli…" and another entry "Content-Type: appli…". ╯°□°）╯︵ ┻━┻

Today, when I added an OAuth key, deleted it and added another one, it sent the request with the previous key. FML

I wrote previously:
"This addon was never finished and died a year ago. It seems the developer lost interest, given it was incarnated ca. 4 years ago and last commit is 1 year ago."
This still holds true.