The landing of patches from two recent bugs has substantially changed the look of about:memory. When you load the page all see now is the following.

Measurements aren’t taken away; you have to click on the “Measure” button for that to happen. Also, adding ?verbose to the URL no longer has an effect. If you want verbose output, you need to click on the “verbose” checkbox.

The motivation for this change was that about:memory’s functionality has expanded greatly over the past two years, and cramming more functionality into the existing UI was a bad idea. There are numerous advantages to the new UI.

No need to control behaviour via the URL, which is admittedly an odd way to do it.

You can switch between verbose and non-verbose modes without having to reload the page.

All the buttons are at the top of the page instead of the bottom, so you don’t have to scroll down.

You can trigger a GC/CC/minimize memory usage event without having do a memory measurement.

When you save reports to file, it’s clearer that a new measurement will be taken, rather than saving any measurements that are currently displayed on the screen.

The buttons are grouped by function, which makes them easier to understand.

There’s also the “Load and diff…” button, which lets you easily find the difference between two saved memory report files. Here’s some example output taken after closing a tab containing a Google search result.

You can see that all the measurements are negative. The [-] markers on leaf nodes in the tree indicate that they are present in the first file but not in the second file. Corresponding [+] markers are used for measurements present in the second file but not the first.