In the past I used to think why Firefox can be slow when it's loading pages. Now I've found something that makes a noticeable difference: the addon "ghostery" that blocks "trackers," which their website defines as
page elements - scripts, pixels, iframes, etc. - that are normally invisible to the user. What "trackers" seem to be all about is marketing, and apparently they've been wasting a lot of my time.
What's especially interesting to me is a list that shows the trackers that have been blocked for each website I visit. For example, the intro page for cnn.com includes 20 such unwanted visitors with names like "Audience Science," "Dynamic Science," and "Scorecard Research."