Mozilla Firefox is a Free and Open Source web browser.
It can have many add-ons which give it extra capabilities.
Mozilla Firefox can be used in anonymous browsing (see The Onion Router). However, it is known that Firefox reveals computer's uptime in TLS (SSL) "Client Hello" packets allowing investigator correlate anonymous and non-anonymous traffic .
This bug affects Firefox 2 (all versions) and Firefox 3 Beta3.