You don't have to do a complete re-install of Firefox. However, what you do have to do is to go into your Home folder, press CTRL + H to show all hidden folders, and delete the ".mozilla" folder. That will essentially delete all your bookmarks, extensions, configurations, etc., but it'll save you the hassle of having to re-install Firefox through Synaptic. The next time you open Firefox, it'll create a new profile for you.
The next best idea is to get a virus scanner, like ClamAV (available in the repos), and scan your entire Home folder. As far as I know, everything under "/" is read-only without root access with the exception of "/home", so as long as you're running as a normal user, your system should not have been compromised unless you did something stupid with root access, but your Home folder may still be contaminated since viruses that enter your system would have read and write access to that folder, so once you get ClamAV, update the virus definitions, and preferably grab Clamtk (the GUI front-end) if you're uncomfortable running a virus scanner from the Terminal, and scan your Home folder recursively. Delete anything nasty that pops up, and you should be good to go.
Debian Testing x64/LM9 Main x64/Windows 7 x64 - LG R580 laptop w/ Intel Core 2 Duo T6500 2.1 GHz, 4 GB DDR2 RAM, Nvidia Geforce G 105M, Ralink rt2860 802.11n, 300 GB WD HD 5400 rpm