Google Inbox, an email app built on top of Gmail for iOS, Android,  Google Chrome browser on a computer and android wear watches, currently needs an invitation to join. If you’re a desktop user, you need to be using Chrome browser to access the Inbox website, if you try to access it using Firefox or other, you will  be informed to download chrome by showing a message that ‘ Inbox only works in Google Chrome. More browsers coming soon.’ (check the screenshot below). 

Inbox by Gmail only works in Chrome message in Firefox

Enabling Google Inbox in Firefox with an add-on

If you’re a Firefox user, who wants to use Google Inbox in it, you can by installing Enable Google Inbox add-on, no configuration is required, install it (works with Firefox 26 or later versions ) and start using Inbox by Gmail.

using Inbox in Firefox

Download Enable Google Inbox extension for Firefox

Why Google Inbox doesn’t work in Firefox?

Inbox team has the reasons for blocking their app on other browsers, for instance, if you consider Firefox, the reason for the exclusion was a performance issue with accessing sparse arrays with huge indexes, read this bugzilla entry for more details.

You can also read the explanation given by Inbox by Gmail team software engineer on Reditt in a single thread.