Run your anti-virus's scan feature. If you can download it, maybe run Internet Explorer if Firefox won't, run
Malwarebytes which is very good at finding malware. Sometimes one will catch something the other misses.
Download Firefox ONLY from
https://www.mozilla.org/en-US/ Download Adblock Plus ONLY from
https://adblockplus.org/en/firefox or
https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/ (Or through Tools/Add-ons in the Firefox top menu.) There are other honest sites but no question about these.
Uninstall Firefox completely then reinstall, should reset options to the default. Not sure installing over an existing version will do enough of that to fix things. Thinking not.
Bad guys will emulate the official page and if you download from there they add all kinds of crap to the download. Guessing that's what happened to you. I got caught once and I'm supposed to know this stuff.
If your anti-virus scan and Malwarebytes doesn't fix things get on Internet Explorer (which I keep installed as a backup for Firefox) or your laptop, Google enough clues to narrow it down and you'll find sites that will tell you how to fix the problem. Don't pay for anything, the solutions are free but some will try to talk you in to buying their product.
BTW, you can crash a program using Windows Task Manager which is where a single CTRL-ALT-Delete takes you on this machine. Terminate firefox.exe, you can crash your machine terminating some processes but a re-boot will bring it back ok.
