1) Performance
These might be select regressions (Google Maps, Slack - so I'll skip the bugzilla references), but every-time a user has to open Slack or Google Maps or Google Docs in Chrome, they are that much more likely to switch from Firefox (ultimately due to Sync issues: bookmarks, history) it just makes so much sense to use a single browser.
2) Developer Tools
I don't know how it is now, but half a year ago when I tried to switch to Firefox Developer tools (again for the Nth time) it would take 2-4 seconds for the console to open. Comparatively Chrome Dev tools opened < .5. As a result even trying to use Firefox for everything else, I still end up in Chromium almost daily.
3) Sync/Mobile
Using Google Chrome has distinct advantages when syncing with an Android device and other Google services (performance issues aside) - most of the things that I considered to be an advantage in Firefox mobile (the top button with <number-of-tabs> for tab switching) Chrome has actually copied.
Minor point - I'm almost sure there is some somewhat underhanded user-memory choice (it's just too smart and annoying to be a bug) - because as someone that is often in foreign countries my Firefox (where I am signed into Gmail btw) always tends to give search results in the native language search, while Chromium (where I'm not signed into anything displays English ones).
Over all I really think Mozilla should focus on getting an advantage on 3) -- I don't think Google and definitely not pocket (which I wish they would unbundle from Firefox) provide a good service for syncing information / personal links / knowledge. There was a browser concept that floated by HN some-time ago allowing users to organize their tabs into sessions / subjects - that could be great! (e.g. check a button to sync only my places/people/events window to my phone and leave my work stuff out).