Hello Clotide,
The Basis steps for you ..
1) Re-Boot your PC and try to open the FireFox
2) Go to "ADD / Remover Programs" and try to repair it..
3) In the background the Files needed for the firefox to open would have got deleted when the ANtivirus software is trying to clean the virus..!
In general...
Keep your
1) windows updated
2) Anti virus updated
3) Browser( firefox ) updated with latest path and version.
Regards,
Sheltan T T