They've been available natively on Nightly with some basic UI support since they were first implemented a decade ago. Some other pre-release builds have them too but it's been too long and I cannot remember which ones did or didn't.
Edit: looks like I enabled them manually and couldn't remember/find it, it's in about:config under the name "privacy.userContext" instead of "containers".
The addons added extra functionality but everything they've listed in this announcement is already there. Sounds like they're just flipping these two flags by default.