You get a bug that a certain feature is crashing Firefox, or slowing it to a crawl. You can't do feature detection, because Firefox reports the relevant features are supported. So you code up a less-efficient behavior that avoids the crash, and only use it for the Firefox user agent.

Months or years later, Firefox improves the problematic behavior, and now performs much better when you switch the User Agent to Chrome and get the efficient path. Nobody at Google is paying active attention to that code anymore, so it remains in that state until someone notices.

The idea of slowing down Firefox based on a User Agent check would be totally crazy to anyone I worked with at Google, and you'd be immediately reported to legal and HR if you tried to get such a thing through code review...