In the late 2000's there were many companies who launched products based on XULRunner (Miro, Songbird, Joost, Tom-Tom...), but when they encountered bugs in the platform Mozilla's response was always "bug doesn't affect Firefox, we don't care". Even if you made a PR yourself (actually a patch at the time), good luck to get it merged if Firefox wasn't impacted.
Note that XULRunner was called this way but you didn't have to use XUL, you could use HTML for UI just like in Electron today.
If Mozilla saw the potential in their platform as a development platform for third party, it might still exist and have a bigger market share than Electron today. Maybe GitHub would have used it instead of developing electron, actually.