Hi sofia,
Finding malware in Firefox’s cache doesn’t necessarily mean the malware is active. It can be an exploit- usually something which is not a risk if you have the latest version of your browser- or a file you have downloaded- only a risk if you then click on the file in the download location when the download is complete.
You can move the file to the chest (quarantine) when avast detects it, or use CCleaner to clean up an delete all temporary browser files:
http://www.ccleaner.com/
