I have 17 instances of firefox running. I want all but one stopped.

How do you tell?

(Amount of processes in task manager is not representative of the number of instances launched.)

So you know exactly what I'm talking about but you give me a non-answer reply. Thanks for nothing. 17 open Firefox processes running in Task Manager is eating my memory.

You want only one instance running. You most likely have. The number of processes (parent, content, graphics, extensions etc.) is directly related to what you're browsing, not how many browser instances (profiles, channels) you run. Every instance will have multiple isolated processes.

(If you want only one single process, the browser won't work. The first ~10 are needed by the essential browser functionality, the subsequent ones are for every addon and page origin.)

In other words, the number is not something you decide, and any arbitrary number of them is also not a reason for resource drain. The content spinning up these is.

If you want to look into resource consumption per process, check the built–in Task Manager - see whether tabs or extensions are slowing down Firefox in about:processes to tell you which of the pages or addons are making up the load and decide whether you want to keep using them.