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Firefox uses some of the chromium code/libraries for the sandboxing on Windows.

https://wiki.mozilla.org/Security/Sandbox/Specifics

I thought one of the main point of Firefox would be to not do this :D

Chromium contains a really solid implementation of OS process sandboxing, which is rather secondary to the bits of building a web browser that we need competition on. It could very reasonably be spun out into its own project, but that takes time and effort so it stays part of Chromium.

I know they use the v8 regex engine: https://hacks.mozilla.org/2020/06/a-new-regexp-engine-in-spi...

My one run-in with this has been that Firefox and Chromium both use libwebrtc, which is managed by the Chromium project as far as I can tell.