The big difference is that both Chrome and Firefox update very regularly, and a high percentage of users keep at least reasonably up to date - the biggest issu

The big difference is that both Chrome and Firefox update very regularly, and a high percentage of users keep at least reasonably up to date - the biggest issue was really that IE had massive market share and was also not updated. We use a lot of chrome-specific APIs, and they allow us to achieve things that otherwise we couldn't (e.g OCR running in the browser through Native Client with no performance penalty vs a native binary). We're often implementing improvements with APIs that are a few weeks or months old. We do test on other browsers though and make sure things either degrade cleanly or we provide an alternative implementation.