Red Hat and Mozilla are the biggest ones I know of.

Red Hat is moving to Jira (sadly) for RHEL. However we're still using BZ for Fedora and upstream projects, and likely will be for a very long time to come.

Do you know if it’ll still be publicly accessible? Browsing the BZ is usually how I gauged internal project timelines.

I certainly hope so.

Mozilla is using a mix of Bugzilla, Jira, Github Issues, and $DEITY knows what else, depending on the team or project in question. A good rule of thumb is that for any major system (code hosting, bug tracking, chat, wiki, CI, etc) Mozilla has at _least_ two- one homegrown and/or OSS, another a proprietary service- and a simmering battle over which should be used.

Most of Firefox engineering was still on their fork of Bugzilla (https://github.com/mozilla-bteam/bmo) when I left back in 2021.

Firefox team use it I think

Eclipse

In process of migrating to a self Gitlab by 2024-2026 [1].

[1]: https://www.eclipse.org/lists/eclipse.org-committers/msg0135...

Not mentioned yet. Proxmox, KDE.

Gentoo Linux

It's right there featured on their home page:

https://www.bugzilla.org/about/installation-list.html

FreeBSD is missing! I know, I could send a PR.