What is offline website data

Personally, I see from Molly's comments, you are concerned about the whole infrastructure - logs etc.

The first thing,

Do not try at work or any machine you do not own.

You simply do not know what monitoring tools are installed, there could be keyloggers, or ANYTHING that sends everything to a remote location completely outside of your control.

Next, as for your actual machine, you really want to run all the usual tools such as CCleaner to clear out your local machine from every day objects.

What does clearing firefox cache do

This should work well, but there is always a remote chance someone can recover your files - You may want to look in to either trashing your hard drive, or doing a complete wipe with something like DBAN.

Lastly, infrastructure - Logs are kept at many different levels.

Your ISP will have various logs that can contain information on sites visited, your routers DNS can have a cache (but it is usually wiped after a few hours)... Your best bet is to use a web based proxy service or even better TOR.

However, if someone really wants to trace what you h