The following Raptor tests are intermittently (quite often) not working on Fennec:
I tried several things including increasing the page-load timeout to a huge number like 3 minutes; changing the 'onload' to 'addEventListener'; and changing what measurements are being taken; but none of those had any effect.
I do notice that if you manually select the Raptor test tab on fennec and then manually choose to refresh the tab in fennec, then the page loads fine and the measurements are retrieved and the rest of the page-cycles work great.
Without that manual refresh it looks like for some reason the Raptor extension measure.js is not being injected into the test tab. This to me points to an issue with fennec itself, not loading the page on the first attempt perhaps.
Also tried running Raptor on fennec in the default tab (instead of opening a new tab; this is how the tests run on the geckoview example app and reference browser, just in the default tab) but the URL won't load on the fennec default tab because of the fennec start page.
Note that another issue in Fennec is the create tab API doesn't make the new tab active by default. Even tried passing in 'active = True' to the tabs.create API (which shouldn't be required as it is supposed to be active by default) but no effect. So when running the tests locally if you want to see them you need to manually select the 2nd tab in Fennec just FYI.