At present, I contribute to Bugzilla Project. I also occasionally review the patches. I have been contributing since past one year.
I had also done a GSoC with Bugzilla, working on the new-bug page, which was aimed at making a faster bug filing page. More details about that here: http://blog.sebastinsanty.com/tags/gsoc
I briefly worked with Treeherder, in introducing the GraphQL APIs to replace the REST APIs. This work is still in progress.
Also almost a year back, I was an active contributor to Firefox, working on security and inspector tools.
Sebastin is a Firefox contributor who helps out by fixing Privacy & Security bugs.