Where did you get this installer from? Only ever download Firefox from Mozilla's website,never anywhere else?

Try getting Firefox from; Download Firefox For All languages And Systems {web link}

teguh_broadway said

I use Linux Mint Cinnamon 18.3 64-bit. I installed Firefox 57 via Linux Mint repository. Yesterday I scanned my HDD by BitDefender for linux and found virus infection on my Firefox files. It could be a virus intrusion on Mozilla or Mint server. It would be better if Mozilla checks his server.

Mozilla does not make the Firefox packages like one you use provided by Mint. However this appears to be BitDefender making a fuss about something in the Cache where it cannot do any harm. More so since you are using Linux and not Windows.

Firefox cache file was infected with a virus