Firefox dev edition browser test errors

In the past few days `browser.test_alloctext` and others fail on `tempCtx.measureText is not a function / undefined`. That seems to not be a flake, but I can't reproduce it locally even with the same browser and flags. Perhaps it depends on OS support somehow and/or how it renders there.

My best guess is that this is related to offscreen canvas perhaps, which is still experimental, and which that test can use (`TTF_Init` will try to use one).