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.

I don't use Nightly and I've been using containers for I think that long without any extension for it.

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".