Firefox didn't generate that dodgy message. It was malware that was already on your computer that made a call upon your default browser (which could have been Firefox, Chrome, Internet Explorer, Safari or anything else) to offer you the dodgy link.
Firefox is perfectly safe (otherwise I wouldn't be using it right now!) and you can simply reinstall it from here:
https://www.mozilla.org/en-GB/firefox/new/
Before doing that though, it might be worth seeing if it's simply the shortcut to Firefox (rather than the program itself) that's disappeared. Go to 'Computer' and double-click on your C drive. Double-click on Program Files. (If you've got a 64-bit version of Windows you'll need to double-click on 'Program Files (x86)' instead). Double-click on 'Mozilla Firefox'. Double-click on 'Firefox'.
If Firefox opens normally then it's just the program's shortcut that's missing. Go back to where you were before and right-click on Firefox, selecting 'Send To > Desktop (Create Shortcut)'.
If Firefox fails to open normally then reinstall it.
(I'm definitely off to bed now, so any further responses will have to wait until tomorrow. Sorry!)