Multifox – is an awesome and very useful plugin for me. If you need to login multiple accounts at the same time, what did we do naturally, we used multiple browsers. But with this plugin on Firefox, you can access as many accounts as you want.
Mozilla Addons link -> click here
But if your browser’s version doesn’t support the plugin, you can try here with an old version from their website Getmultifox.com
For details and getting started check here
N.B.: Chrome already have this feature builtin on Settings > People > Add preson