Mozilla uses Trello quite a bit for project management (and Bugzilla and GitHub for bug tracking), but some teams are experimenting with Airtable. It's like a cross between a Google Sheets and Trello. It's not as approachable as either of them, but it is more robust. You can create spreadsheets with real schemas and foreign keys, comment tracking and notifications like Trello, and multiple data views like Kanban or calendar.
Bug tracking at Mozilla is a challenge. Some projects have their code in Mercurial and bug tracking in Bugzilla. Others have code and bug tracking in GitHub. And some projects have dependencies that bridge both, so there is no good way to see a global view of relevant bugs in Bugzilla and GitHub. Some teams experimented with Jira plus some custom extensions that provided a unified view of Bugzilla and GitHub, but it didn't go far because it was a lot of work to maintain and developers would still be using Bugzilla and GitHub directly.