Basically I'm already used to high memory usage by Firefox - 300-400 MB is normal for me. However, after upgrading to FF4 RC, I got very big problem with memory usage: if I leave FF running idle for long time, it eats up a lot more memory than usual. During the night it had taken 2.6 GB! On another occasion it took only 1.5 hour for FF to eat up 1,6 GB although it started only with some 350 MB. Also, I've checked memory usage with 'procexp.exe' for firefox process and saw that once I refresh some pages (let's say about:memory) couple of times after some period of inactivity, memory usages normalizes (goes from ~700 MB to some ~400 MB). Another situation is where memory usage graph looks like zig-zag (memory goes from 300 MB to 450 MB in ~8 min and then goes back to normal; repeat). Actually this was happening while I wrote this problem description. So anyone? Have you the same problem? Can you point me in right direction to solve this? Or should head to bugzilla to file a bug?