Was this your first experience with Firefox? Sorry to hear about the problem, it may be that there was already some software on your system that injects add-ons into Firefox.

Could I suggest you simply don't do anything with Firefox for the moment but try to clean your system a bit. Here's what I suggest:

(1) Open the Windows Control Panel, Uninstall a Program. After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help surface undisclosed bundle items that snuck in with some software you agreed to install. Be suspicious of everything you do not recognize/remember, as malware often uses important or innocent sounding names to discourage you from removing it. Take out as much trash as possible here.

(2) You can supplement your regular security software with some of the free scanning/cleaning tools listed in our support article: Troubleshoot Firefox issues caused by malware. These on-demand scanners take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.

(3) Before trying Firefox again, replace the program folder as follows:

(A) Rename the folder

C:\Program Files (x86)\Mozilla Firefox 

to

C:\Program Files (x86)\BadFirefox

(B) Download a fresh installer for Firefox 38.0.5 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(C) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Any improvement?

(4) Open Firefox's Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons
• in the Windows "Run" dialog, type or paste
  firefox.exe "about:addons"

In the left column, click Plugins. Set nonessential and unrecognized plugins to "Never Activate".

In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

After, this add-on will help.

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