[fix bug 1073239] Update support link on Firefox release notes

Update support link on Firefox release notes depending on product.

https://bugzilla.mozilla.org/show_bug.cgi?id=1073239
This works, but I think it would be more readable if you use a dict, like so:

```python
support_url = {
    'Firefox for Android': 'https://support.mozilla.org/products/mobile',
    'Firefox OS': 'https://support.mozilla.org/products/firefox-os',
    'Firefox': 'https://support.mozilla.org/products/firefox'}
return support_url.get(product, 'https://support.mozilla.org/')
```
You might also consider defining the dict at the module level outside of a function and just use `support_url.get(product, 'https://support.mozilla.org/')` on [line 550](https://github.com/mozilla/bedrock/pull/2324/files#diff-784e0d4de42af29595e70315a6be5337R550) instead of defining a `get_support_url` at all.