> This is proving just as clearly that the more you use a web browser, the more likely it is that you will block ads.
No. That's the point of using propensity matching in a longitudinal dataset. What this 'proves' is that 'when two new Firefox users who use a web browser an equal amount (as defined by a number of variables like time-in-browser or total-TLDs or total-URLs) are compared, one of whom installs adblock and one who does not, afterwards, the adblock user subsequently increases their web browser use compared to the non-adblock user'. As they put it:
> Restricting to new users who installed Firefox during a specific time period, we select a test group of users who installed an ad blocker, and a control group that did not. The selection is performed using a matching technique applied to baseline user features, which allows us to infer a degree of causality from the observed effects. We then compare test group Web usage after installing the ad blocker to control group usage over a comparable period, controlling for baseline usage levels. This is sometimes referred to as analysis of covariance, and is closely related to difference-in-difference analysis...This is achieved using matching. Each test group member is paired with a similar member of the control group, where similarity is determined based on features observed during the baseline period. This ensures that the groups start off “more or less the same”, e.g. that one group is not biased towards more engaged users...Taking into account the matching and adjustments made to ensure comparability between the groups, we find that installing an ad blocker does in fact have a strong effect on average Web engagement. Users who installed an ad blocker were active in the browser for around 28% more time on average, and loaded 15% more pages (URIs), controlling for baseline activity.
(Which of course still doesn't prove causation, because there is 1 remaining possibility: some third factor caused both the adblock installation and then later in time the increased web browsing. But it doesn't prove what you say it does.)