Postman is not available for Firefox but there are a few alternatives with similar functionality. The most popular Firefox alternative is 
RESTer, which is both free and Open Source. If that doesn't suit you, our users have ranked 47 alternatives to Postman and three of them are available for Firefox so hopefully you can find a suitable replacement. Other interesting Firefox alternatives to Postman are 
HttpRequester (Free, Open Source) and 
Resting (Free, Open Source).
Postman is a collaborative platform for API development. If you're looking for more info about Postman like screenshots, reviews and comments you should visit our info page about it. Below you find the best alternatives.
A REST client for almost any web service (Firefox and Chrome extension).
HttpRequester is a tool for Firefox for easily making HTTP requests (GET/PUT/POST/DELETE), viewing the responses, and keeping a history of transactions. This tool is useful when doing web or REST development, or when you need to make HTTP requests that are not easily done via the browser (PUT/POST/DELETE).
Resting is a rest client. The mission of Resting is to simplify daily work of developer in testing and analyzing HTTP/Rest requests. Resting takes inspiration from Postman with the goal to be light and focused on the management of saved requests.
