Reasons:
- Firefox got bogged down with just a few tabs open, and caused beachballs (OSX/macOS) systemwide. Chrome was snappier and didn't harm my system's overall responsiveness with several times as many tabs open. This was the main reason.
- Dev tools. Liked Chrome's better.
- Profile handling was, at the time anyway, better.
- IIRC Firefox didn't do per-tab crashing at the time, while Chrome did, which aided overall stability.
Advertising had nothing to do with it. Chrome was just way, way better, especially its (apparent, which is mostly what matters) resource footprint.
Now I'm mostly on Safari, even though it's the worst mainstream browser, just because I gain 1-2hrs of battery life using it over Chrome or Firefox.