There is a rash of that going around and the solution is not yet 100% clear. I suggest first trying the reinstall this way:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean -- and do not contain alien code files. As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(1) Download a fresh installer for Firefox 37.0.1 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox (if applicable).

(3) Rename the program folder

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

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

Any improvement?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Some additional suggestions:

(1) Open the Windows Control Panel, Add/Remove Programs. After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help surface innocent sounding bundle items that snuck in with some software you agreed to install. Take out as much trash as possible here.

(2) Check whether Firefox has a non-standard connection setting. You can do that here:

"3-bar" menu button (or Tools menu) > Options > Advanced > Network mini-tab > "Settings" button

The default "Use system proxy settings" should piggyback on your Windows/IE "LAN" settings. But you can try "No proxy" to see whether that makes any difference.

(3) Our support article "Troubleshoot Firefox issues caused by malware" lists cleaning tools other Firefox users have found useful. These on-demand scanners are free and 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.

Success?

By the way, where does it appear on the Troubleshooting page? Could you copy/paste that into a reply? There's a pair of "Copy to clipboard" buttons in the upper left on that page. You could use either of those and paste into a reply. You can clear the parts after the problem, as those probably aren't relevant.