Firefox maintains a cache where it stores temporary files such as web pages and other online media that you have downloaded from the Internet. Your Firefox cache is a folder on your local computer.
When you are performing a scan for viruses, some anti-virus scanners may find a file in your Firefox cache that it believes is a virus.
Note: It is unlikely that you have been infected.
Firefox does not launch some types of files that are downloaded into the cache, which prevents most viruses from propagating to your computer. For a virus to spread to your computer, it must be launched by opening or double-clicking on the file. Unless you have launched the file from inside your Firefox cache, you probably did not get infected with a virus. It is unlikely that Firefox has opened files of a suspicious type.
Additionally, anti-virus scanners may incorrectly identify cached files to have a pattern matching a virus. To anti-virus scanners, some files that are otherwise harmless may be mistaken for viruses.
If you are unsure if the file location is in your Firefox cache, you can find the cache location by typing about:cache in the Location bar, and pressing Enter.
Because the files in the cache are temporary, you can permit the anti-virus scanner to delete the file if it's in your Firefox cache.
You can also clear your Firefox cache at any time. Click the menu button
and choose Options.Preferences.
Select the Advanced panel.
Open the Network tab.
Click on Clear now.
Close the about:preferences page. Any changes you've made will automatically be saved.