To pile up on top of bug 1539518, we could also have an API to instruct the test helper to display the "dominators" of any given JS object of DevTools.
Dominators are the objects which are referencing another one and justify why the given object is still being allocated.
Such information is crutial when it comes to understand why an object is leaked.

Bug 1539518 will help narrow to which particular object is being leaked, and this feature will help understand why in a very efficient maner.

This API, to be enabled locally and manually via DEBUG_ALLOCATIONS=1 env variable,
helps knowing why a given JS object it being kept alive.

You also have to manually hack your codebase to tell which precise object instances
you want to trace.

You can see the new helper output by running:

$ ./mach mochitest devtools/client/framework/test/allocations/browser_allocations_target.js  --headless

[...]

 0:16.84 GECKO(646818)  # Tracing: Object@Pool.js:27
 0:16.84 GECKO(646818) ### Path(s) from root:
 0:17.17 GECKO(646818) - other@no-stack:undefined.WeakMap entry value
 0:17.17 GECKO(646818)  \--> LexicalEnvironment@base-loader.js:160.**UNKNOWN SLOT 1**
 0:17.17 GECKO(646818)  \--> Object@base-loader.js:155.g
 0:17.17 GECKO(646818)  \--> Array@target-mixin.js:38.objectElements[0]
 0:17.17 GECKO(646818)  \--> Object@target-mixin.js:46.seeMyCustomAttributeHere
 0:17.17 GECKO(646818)  \--> Object@Pool.js:27
 0:17.17 GECKO(646818)  # Tracing: Object@Pool.js:27

https://hg.mozilla.org/mozilla-central/rev/bea832932e37