Did you place a check mark in the "Remember me" box, when you logged in? I'm not sure, but that might be your problem. You could try logging out, removing the check mark and logging back in.

If that doesn't work you may want to try resetting your browser to the default settings (there should be an option for this), then clearing your cache, history, and cookies.

If all else fails, you could try a different browser. Chrome seems to work well with the site these days.