FF doesn't use ActiveX (unless you specifically install an extension to do so, which really just wraps IE in the tab as needed for ActiveX controls).
Try some of the online house call scanners (you'll need IE cause they use ActiveX, sadly).
Panda and
Trend Micro provide decent ones.
If this doesn't solve the problem, get
spybot search and destroy, update it. Reboot into safe mode and run it. It might be able to delete the malware. If not, run your virus scanner, still in safe mode.
If this still doesn't work, get
hijack this, run a scan, and post the log on a security forum (such as on dslreports.com), along with all info you can provide, like the OP and what you have tried thus far.