Install the 'Multifox' on Firefox Nightly with 'browser.tabs.remote.autostart' value 'TRUE' 2. Sign into gmail account on Default windows of 'Multifox' 3. Locate the 'Multifox' add-on on Menu bar 4. Click on it to choose 'new' option to open a new window 5.

Using websites with multiple accounts at the same time is made easy. Create an independent tab with a click of a button. Use one browser with multiple active sessions per site. Log into multiple.
Multifox 3.2.3 hultmann released this on Feb 18, 2017 For some reason, Mozilla removed Multifox from addons.mozilla.org (maybe due to e10s incompatibility). I've uploaded the latest version on Github for those who missed it.
Steps on how to add Multifox: 1. Go to Google search and type Multifox addons and hit enter 2.
Using websites with multiple accounts at the same time is made easy. Create an independent tab with a click of a button. Use one browser with multiple active sessions per site. Log into multiple.

Some time ago, we talked about CookiePie, a Firefox add-on which allows you to use a new session so that you can log in to the same service with multiple accounts (such as multiple Google accounts, etc). But CookiePie doesn't always work and it breaks the cookies if you move the tabs around.
Multifox is another Firefox extension for logging in to multiple accounts on the same site which I like because it doesn't messes with your cookies and I was never logged out from both accounts like it sometimes happens with CookiePie.

Multifox works by adding an entry to the tabs context menu called 'Open in a New Identity Profile':

and by clicking this, a new window will open so you can now log in to another account. The new windows will have a number displayed in the URL bar:

Multifox Addon

Install Multifox (don't worry about the site being in Portuguese, the add-on is in English).