I think Blink is the rendering engine. Chromium is Chrome without Google branding
I think that the problem with Firefox right now is that it does not have something unique to offer. Before moving to the new plugin model, there were plugins that were not available on Chrome which was available for Firefox : DownThemAll, Tab wheel scroll etc. But in their desire to chase Chrome they threw it away.
I also seem to have an annoying bug in mobile where after a while long tap on link no longer opens the contextual menu. I wonder if I should report it since last time I reported a defect it turned out to be a problem with uBlock and this one takes a while to reproduce.
I think it might be a good idea for Mozilla to invest in good extensibility. It helped them a lot last time and is therefore a battle hardened tactic