Interesting... Can we get a screenshot of the issue? Is there a reason for that version of Firefox? Depositing... So when you open the browser is it downloading? More detail please.

IF you mean the Firefox Cache then anything in the Cache is harmless if left alone and get overwritten or deleted. Files are added to the browser cache when you retrieve a web page, so that's most likely how the file got in there.

https://support.mozilla.org/en-US/kb/Firefox+cache+file+was+infected+with+a+virus

Whatever unnamed virus or malware scanner you are using on Linux may also be doing false positives or making something simple as more dangerous.