Im not sure what exactly you mean... Browsers are not anti-virus programs. windows 8.1 has a very good anti-virus built in so you shouldn't need anything else. It's more important to make sure you keep windows, Firefox and all your plugins up to date. Always always Always install all updates.

Malware/adware can easily get into Firefox if you're not careful with what you install. It gets on your computer after you have installed a freeware software (video recording/streaming, download-managers or PDF creators) that had bundled into their installation

Please note that Firefox isn't meant to be used in place of antivirus software. More information can be found at the How does built-in Phishing and Malware Protection work? article. If you don't have an antivirus running, I personally suggest one of the following

• Avast
• AVG
• Avira