Add content for /firefox/tiles page

Bug 1059518

Known issue: content is still missing for the last FAQ question ("Where can I learn more?"). The rest can be reviewed.
I think it's safer to regroup these 4 in just 2 strings.
```
{{ _('Nothing is ever personally identifiable. In other words, we don’t know who clicks on what.') }}
{{ _('We just know the total number of views and clicks an individual Tile receives. This helps us deliver the most interesting and relevant content.') }}
```

If strings are too long, we risk to invalidate an entire paragraph if a simple word changes. At the same time, if it's too short it might get hard to localize it and glue together the sentences naturally.

Rest looks good, thanks :+1: