[Feature Req] Update Firefox screenshot in the desktop installer slideshow

[lucasleung8]

The image in installer slide show should be updated because for 3 years now Firefox redesigned its interface - pictures below. This is important to ensure Mint doesn't seem outdated, especially to new users who open up Firefox after installing the OS and see the inconsistency.

https://blog.mozilla.org/en/products/firefox/fresh-new-look-for-firefox/

Current:

Expected:

[lucasleung8]

The windows 7 and Internet Explorer screenshots are also outdated, per linuxmint/ubiquity#71

[clefebvre]

Closing since this is tracked on ubiquity.