Unique user and multiple browser

I’m interested in the unique visitor statistic. That’s why I want to know, how unique visitors are tracked, when users use more than one browser. Because every browser has ist’s own “cookie-folder”, each visit using another browser a cookie is set. Is this fact considered in the evaluation of unique visitors?

Unique visitors in piwik: http://piwik.org/faq/general/#faq_43

if you use several browsers, piwik will count several unique visitors.

The FAQ states:

If the visitor doesn’t accept cookie, a simple heuristic is used to try to match the visitor to a previous visitor…

Does this heuristic get applied when using session cookies and what’s the chance of the unique visitor statistic being anywhere near valid, if we’ve set Piwik’s cookie expiration to be immediate (essentially session)? My organization is not allowed, without an exception signed by a high level official, to use persistent cookies. I need to know so I can inform a user if they should base an evaluation criteria on Piwik’s unique visitor tracking.

This heuristic is used at every request. It will detect unique visitors correctly if the visitor didn’t change his resolution, browser, or IP

Where can we access the algorithm used for unique visitor identification? Both with and without cookies.