[WiseWeasel]

You can account for those factors by parsing only certain bits of the User Agent string, and allowing for the addition of fonts to the list (most typically don't uninstall fonts). With the plugins, you can ignore the version number and just go by the names. There are bits like browser name, OS name, screen resolution and the presence of all previously detected fonts and plugin names that you can be pretty sure won't change for most users. As long as you can uniquely match by certain factors, it'll be enough to link you to your previous session.

For a purpose like ad tracking, the period of time you need to track people is likely pretty short, as in from when they click on a banner or text link until they complete a purchase, so you can compare lots of data points to identify them. If you need to track for longer periods, like to retarget an ad to people who have completed purchases for x, then you would need to compare fewer, more stable points and hope you find a unique match.