While we do use a server-side cache, it expires things in minutes; and it gets restarted daily. If you're seeing something older than 24 hours, it's definitely not BG itself.
It could be some equipment between you and BG (a proxy, something the ISP is doing, etc.) misbehaving. I'd still try clearing Firefox's cache (we are a very old-style site, so it's possible a Firefox bug could affect BG but nothing else you use -- e.g. we don't explicitly ask for client-side caches to expire, unlike most modern sites).
