Add 90x90 icon for Firefox

The [Firefox validator](https://marketplace.firefox.com/developers/validator) complains that there is no 90x90 icons:

```
90x90px icon should be provided for Firefox OS.

Warnung: An icon of size 90x90 should be provided for the app. Firefox OS will look for this icon size before any other.
```

Couldn't find that written anywhere else, but it's a warning less. ;)