Thanks! I have used about:memory and 'CC', 'GC', etc, but they didn't help. I will see if this is a problem with the new version. At least older versions (25 or before) all had the similar issue.
Those buttons aren't silver bullets. If you file a bug and include the contents of about:memory (use the "measure and save" button) and CC me or mccr8 there's a decent chance it'll get looked at.