Firefox Browser doesn't send such messages that is from whatever sites your going to that is using the Browser to send you fake messages. The fact it says to turn on virus defender should tell you there no such software like that. You need to use your system A/V and scan for issues otherwise a computer shop is going to need to look at this problem.

^ignore the comment above

I've been getting messages labeled Moxilla Firefox that tell me to "click here to fix the error now" or "turn on your virus defender" for issues like virus attack, trojan virus, critical alert, etc. Is this a scam or do i need to click? I've been clicking on "Dismiss" to make them go away but they keep coming back. Getting 50-100 a day.

Are these small popups on bottom right corner of the Firefox Window? if so then they could be web push notifications from sites you have allowed to send you notifications from. Unfortunately some sites may send you fake virus alerts instead of useful information. You can revoke these sites.