This is probably referring to some work to use Stencil for self-hosted code. That reduced process overhead by 6%, which is quite a bit: https://bugzilla.mozilla.org/show_bug.cgi?id=1688794

There might have been some other Stencil work that landed in 92 to reduce memory.

There's a description of what Stencil is, and some work related to it that landed in Firefox 90 and 91, here: https://spidermonkey.dev/blog/2021/07/19/newsletter-firefox-...