Performance Monitor is a new internal page in the Firefox browser that displays slow add-on alerts and resource usage of the browser starting with Firefox 40.
One of the things that I like about Google Chrome is the browser's Task Manager as it highlights resource usage of browser components in an easy to access way.
While you have a couple of tools for Firefox that provide you with similar information, they come mostly in form of add-ons that you have to know about before you can install them.
The newly integrated Performance Monitor is an internal component of Firefox which means that you can run it without installing add-ons first.
Just type about:performance in the browser's address bar to get started. Depending on how you are using Firefox at that moment in time, you may not see any data or lots of it.
The performance monitor divides data into performance related information that update automatically while you are using the browser and slow add-ons alerts which seems to use the new slow add-on notification system that Mozilla integrated in Firefox 38 Nightly.
Resource use is sorted from highest to lowest which may already give you a good understanding of what is causing slow downs or memory usage in the browser.
Most of the information provided on the page are designed for developers and not end users as most users probably don't know what jank level, activations or cross-process means and how it relates to the browser's performance.
That's not a big issue however since the data is sorted already so that you can react based on that alone if necessary.
The tool could use additional features though. One would be an option to collect historic data so that you get information about performance requirements over time and not only in particular moments in time.
All in all though this can be a useful feature to all users of the browser. If you notice slow downs for instance or lags while using Firefox, you may be able to tell why those are happening by opening the about:performance page in the browser.
If you see a web page, browser component or add-on listed there at the top, you get all the information you require to do something about it.
For instance, if it is an add-on, you may want to turn it off for a test to see if your browsing experience improves. If it is a web page, you may want to close it to resolve the issue.
It is unclear when this feature will land in stable builds of the browser. If the version is anything to go by, it may take three releases before it does.