Getting Web Replay to work on Linux is important, and crucially so for the current plan for cloud integration (bug 1547081). It would be good to get recording, replaying, and rewinding to all work reliably. The attached WIP deals with most of the changes necessary to get recording to work. This factors out the redirections shared with macOS into their own file, and has some tweaks to the build system, graphics, and sandbox so that the recording process can run without crashing and show graphics for a simple page.
Disclaimer: this work was done on my own time and is not part of my official work at mozilla. If you have a problem with me working on this, please talk to me directly.