The web is a fantastic place where you can have almost everything that you want, and many software that you download are either freeware or shareware needed for your system. But these software often are very harmful for your computer as many freeware and shareware come with adwares/worms or viruses, which your antivirus will not always automatically detect.
DrWeb is an antivirus that is mostly renowned for its up to date list of viruses and the exactitude of the scanning that is being performed. If Firefox has insisted to get a merge with the antivirus, you can understand to what extent the antivirus is a good one. Besides, the antivirus is of a Russian origin (if they have successfully sent a rocket in space, not you imagine for an antivirus!)