can a malicious keylogger Spyware get through Mozilla

You are right to be unsure, because no browser can be 100% secure against malware that is already on the device. A keylogger runs at the system level and sees what you type before it ever reaches Firefox, so the browser cannot block it. What I do is keep Windows updated, run a trusted antivirus, and turn on two-factor authentication on my important accounts, so a stolen password alone is not enough to get in.

If by "get through" you mean users click random things, get tricked into downloading stuff, and then running things because they are promised… games… discounts… sweepstakes… whatnot, then yes, most of malware is self–inflicted by users blindly trusting random promises of malicious actors and compromising their systems manually themselves. So that's something everyone needs to be wary of when running things downloaded from unknown sources.