When offline, Firefox searches in the cache. So if your cache is very big, you will keep for example 5 days of traffic.
If you increase it, you will keep track of more pages.
I'm also using offline browsing but for this I use this tool, which is transparent:
http://www.gedanken.demon.co.uk/wwwoffle/
You set it up to act as a proxy that goes on internet.
You tell firefox to use wwwoffle as your proxy.
It caches a lot more pages, and when you put it offline, you don't even need to tell firefox to go to offline mode. It reaches wwwoffle that searches in its cache for the requested page. You can also perform a search in these pages.
There are some nice other features.
Depending on your distro, there could be an already made binary package
