Not at all - all software allows you to keep it patched and up-to-date.
So long as your operating system has the latest patches installed, you are using the latest browser and you keep your software up to date you should not experience problems.
Most viruses/spyware/malware are known and are blocked by 1. the patches for the operating system and 2. by the latest signatures available from security software vendors.
Firefox is just as susceptible to these threats as any other browser. The key is ensuring that all the software you use is kept up to date, Firefox included.
For everyone:
Check out Secunia PSI. It's a free to use software inspector. It scans the software on your PC and checks it is up to date. If it finds anything that is out of date it provides a link to the files required to bring that software up to date.
http://secunia.com/vulnerability_scanning/personal/