It seems there is some limited form of isolation, but not as strong as on Chromium-based browsers:
https://bugzilla.mozilla.org/show_bug.cgi?id=1565196
GrapheneOS claims that Firefox bypasses some work done by them and upstream Android but they don’t specify how exactly…
They did not ditch WebKit for Blink, they ditched it for Gecko (Firefox), which is a good thing IMO.