Fix Bug 1069663 - mozilla.org treats Ubuntu on ARM as Android

> Generic link text like "here" isn't good for readability and accessiblity. I'd recommend something like `Please <a>follow these instructions</a>` to install Firefox.

I agree that's better than my original suggestion, @craigcook, and no, @kyoshino the specifics of the string were not previously discussed-- I just took a stab at it in my original PR, hoping that someone would suggest something better. How about the following:

```
{{ _('Please follow <a href="%(url)s">these instructions</a> to install Firefox.')|format(url='https://support.mozilla.org/kb/install-firefox-linux') }}
```