PureBrowser Home screen (about:home) bubbles up after some time one of the following texts:
Thanks for choosing Firefox! To get the most out of your browser, learn more about the latest features
...where the last part links to Firefox features page on mozilla.com.
It's easy to customize your Firefox exactly the way you want it. Choose from thousands of add-ons.
...where second sentence links to featured add-ons on addons.mozilla.org which includes non-libre add-ons (see T299).
The bubbling effect seems to potentially fetch additional advertisement texts from Mozilla domain, which (if true) leaks user activities to Mozilla.