Can you download and run some of the cleanup tools listed in our support article: Troubleshoot Firefox issues caused by malware. There also are a couple of specialist malware removal forums listed where you can get better step-by-step guidance on more powerful tools that may not be safe to use without assistance.

Also, in case your installer has been modified from the original, please try this download page: https://www.mozilla.org/firefox/all/ (scroll down to your preferred language)

And... after Firefox gets installed, before you run it, try creating a clean profile that won't be affected by your earlier settings file and extensions.

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder.

Exit Firefox and start up in the Profile Manager using Start > search box (or Run):

firefox.exe -P

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button. I recommend using the default location suggested (i.e., skip the button that lets you change the folder). Then start Firefox in the new profile you created.

Cleaner?? If so, we can point you to articles on restoring your last bookmark backup to the new profile, and moving over any other critical data.

Or you can try to clean your old profile, which might be less work in the end.

When returning to the Profile Manager, you might be tempted to use the Delete Profile button. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.