How to differentiate between different running Firefox instances?

I'm running several Firefox instances side by side on Ubuntu, each with a different user profile.

When each of them has several open windows I get a stack of sometimes two dozen windows, which is very difficult to choose from the specific instance I'd like to e.g. open a new tab in.

Is there an elegant way to differentiate between them? E.g. some about:config trick to add a prefix or suffix to the window title of all windows from a specific profile ? (i.e not a universal change for all instances in a resource shared by all profiles).

The Firefox extension MR Tech Toolkit can, among many other options, replace the "Mozilla Firefox" text that is appended to the title bar to anything you like, which can also include the profile name.

Below is shown how to change it to the name of the profile ("default" in my case).

There is a Firefox add-on called Show Profile by neo82 that worked for my purposes. Below is the screenshot of its Preferences panel.

This answer won't help you since I see you're running Linux, but the question itself is generic so I figured I'd offer an answer for any Windows Firefox users in a similar conundrum.

SysInternals Process Explorer is a useful tool that will show all running processes, and also has the handy ability to identify what process a window is associated with by clicking on it with a target cursor.