We have an assertion being triggered on Pine because of the WebComponents used in Gaia. As per https://bugzilla.mozilla.org/show_bug.cgi?id=1261351#c48 this is relying on something that is not anymore in the WebComponents specs. It means Gaia web components (which are used everywhere) will have to be changed!
There is no direct pointer in the bug to the Gaia code path that is broken (because probably a lot of them) so it might require some time to investigate. It is reproduced though with mulet builds and devices builds. Only in debug mode.
We will have to remove the kind of workaround that we have to bypass that error, so fixing gaia is really important