Firefox has always had problems with memory. If you closed it fairly frequently and had enough system RAM, you might not have noticed them, but if you were pressed for memory space or [particularly] if you left FF open for long periods of time, you were vulnerable. Those issues plagued me badly here because I might leave the browser open for a week or more.
Apparently FF 3 is making splendid progress addressing this shortcoming. It's about time, to be perfectly frank. Here's an account of what has been changed.