I have a problem that Firefox crashes whenever I try to customise the toolbar.

I experienced a BSOD after which all my firefox profile settings were screwed. I then discovered that when I tried to customise my toolbars, as soon as I clicked on 'customise', the customisation pop-up appeared for about 2-3 seconds and then FF crashed.
I decided to completely reinstall FF. I uninstalled then installed the latest version downloaded from Mozilla. No joy. Still getting crashes when attempting to customise the toolbar. A bit of searching turned up this on Basic Troubleshooting
"Some malware programs may target the Firefox application file. The following instructions may help diagnose such a problem:

1. From the menu at the top of the Firefox windowbar, select FileFirefox and then select the ExitQuit FirefoxQuit menu item.
2. Open the Firefox application folder - by default, it is:
   C:\Program Files\Mozilla Firefox.
3. Rename firefox.exe to firefox1.exe.
4. Double-click firefox1.exe to launch Firefox.
5. If Firefox runs without problems, some malicious code may be targeting Firefox (firefox.exe). Scan your computer for malicious code using the links above or your own antivirus or antispyware program."

So I tried renaming to firefox1.exe as suggested and bingo! No crashes when customising the toolbar. So I ran Spybot S&D and Adaware (I fairly regularly run S&D but had not run Adaware for some time). Nothing untoward showed up with S&D but Adaware showed an infection by Win32.backdoor.bifrose which I then removed. Still no joy. FF still crashes when I try to customise the toolbar when running as firefox1.exe but not when running as firefox.exe. However, I am a user of SignupShield, the toolbar for which does not appear when I run firefox1.exe.

So, at the moment, to customise my toolbar I rename to firefox1.exe, do the customisations, then rename back to firefox.exe and run that for normal running. But I'm a bit worried that there may still be some malware lurking that Adaware and S&D have not found. Does anyone have any suggestions what I could try next to isolate my problem?