The future versions of Mozilla Firefox will come with a new feature called electrolysis or e10s in short. This makes Firefox run in multiple process mode in which Firefox browser runs in one process and the web content is rendered in other separate processes. There are several benefits of having multiple processes, for example, the stability, security and resilience etc. But as this is a new feature, some add-ons might not work readily with the e10s and would need an update. The updating of add-on depends entirely on the individual authors and could take months. In the meanwhile, if you want to use the e10s incompatible add-ons in Firefox, you may have to start a non-e10s window in which that add-on would work.
There are many ways to launch a non-e10s window in Mozilla Firefox. If you have already opened a web site and want to switch it from an e10s enabled window to a non-e10s window, then you can simple right-click on the tab in which that site is opened and choose Open in new non-e10s window.
But if you have yet to open a website and plan to open it fresh in a non-e10s window, then you have to press the Alt key once to make the menubar appear and select New non-e10s window from the File menu. This will open a new window without spawning any new process.
Exactly the same goal can be achieved if you click on the hamburger icon (icon that looks like three horizontal lines on the top-right corner) and then click on the New Non-e10s from the panel that shows up.
The main difference between a non-e10s window and the e10s window is that Firefox launches an e10s window in a new process (depending on how many processes are allowed in Firefox) while the non-e10s windows are launched using the existing Firefox process. As you can guess, if a non-e10s window crashes, it might also result in crashing the Firefox browser itself.