Is it a good idea to have a 2nd antivirus security on your computer?

Right now my computer has Microsoft Defender Antivirus that scans every few days to check for any security problems, every time it scans it always shows no threats were detected. Every once in a while I'll get a couple warning pop up boxes saying a threat has been detected, I'm wondering if my Microsoft Defender isn't detecting something during it's scan. Is there a free security app I can download to check for any other problems I may have that my Microsoft Defender doesn't scan for?

 

Does Windows Defender check for malware and spyware? I was just wondering why my browser was starting to run a little slow, also if I click on a link, another web page site will open instead of the one I clicked on, hat's when I started to get those warning boxes that said my computer may be infected, the warning boxes looked fake to me. I'm not sure if this would have anything to do with it or not, over three weeks ago I opened my Facebook and started scrolling down on my friends posts, then this box popped up and told me I was Temporary Block, it's been almost 30 days and I still can't view my Facebook page, I never had anything happen like that before, that's why I was thinking maybe I had more than just a virus somewhere. Below is what I keep getting on my Facebook page, have you ever seen or heard about a block box like this?

 

I have two browsers, Microsoft Edge and Fire Fox. I usually use Fire Fox as my main browser, I'll start using my Microsoft Edge and see if there's any difference. As far as the Facebook Block message, would you know if that would be part of a browser hijack or would that be a Facebook problem, I never heard of a fb block message because you were scrolling to fast..