When hiring at Mozilla, having potential candidates who know open source software is almost a requirement. But there's a huge difference between people that work with open source software and those who work on open source.
About 14 years ago when I started interviewing candidates for open source software, even seeing candidates who knew what open source was could be unusual and seen as a advantage. That's not enough now. These days working with open source software is seen as a base requirement. But that's not still enough.
What I really want to see is that you've worked on open source. Have you:
- contributed to an open source project?
- published an open source project that is used by someone other yourself (or your company)?
- given a talk at a open source conference?
- helped out an open source foundation?
- written some documentation on open source?
- participated on mailing lists with other developers about an open source project?
- dealt with those awesome people who want to help and those trolls who don't?
There's a reason we look for open source developers at Mozilla. It's partly because Mozilla is basically a collection of open source projects with some funding behind it. But also because developers on open source are great at developing code and at working with other people.
Working on open source separates you from those who just use it.