I don't know of any software projects of comparable size and complexity that doesn't have years-old bugs languishing in limbo. This is the normal state of affairs for a web browser, and it's the result of human-factor bottlenecks that aren't easy to solve even for well-capitalized projects. Large projects end up having subsystems that work reliably enough, such that their developers can afford to shift their attention elsewhere. Over time various details fall out of working memory. You get to a point where there is a high up-front cost to making any significant changes to the subsystem because it requires a significant investment of time for people to acquire (or re-acquire) the degree of familiarity that is needed to make such changes comfortably. If this cost can be amortized over a large backlog of feature requests and bugs, so it can keep someone busy for a while, then it is worthwhile. But it often isn't.
A primary software at work is $1,500 to $10,000 a seat (depending on extensions) plus 10% annual maintenance. Our department annual fee is ~$70,000. I have a long list of "vote up this enhancement request or bug to get it fixed" that goes back almost two decades. (It's hard to identify the oldest because the vendor has switched tracking platforms twice.)
