FF on Linux. Don't use maps, but YouTube works as it should (but then again, I only allow youtube.com, ytimg.com and googlevideo.com via NoScript).
It might be deliberate, at least in sense that MS pages never worked properly on Netscape / Firefox / Konqueror /... in the days of IE dominance.
On a 4 year old MacBook Air running Firefox, google maps takes a second or so to open fully, after browsing around in two google maps windows for a while my memory usage is stable at 1.25GB.
Especially Google maps and YouTube. I always have to switch over to Chrome to use these sites because they always bring FF to its knees. I wonder if this is deliberate on the part of Google.