This should help -

https://support.mozilla.com/en-US/questions/861272

You have an item "My Web Search . . ." plug-in which is a well known troublemaker.

You should work through the following removal instructions as far as you need to (Google for information if you have any doubts.)

Courtesy of Helper7677 -

You have an item installed that is considered malware/spyware/adware. To see the Plugins reported with your question, click "More system details..." to the right of your original question or, on the Firefox menu, click Tools > Add-ons > Plugins.

   My Web Search Plugin Stub for 32-bit Windows 

This type of pest is usually installed along with software you download from the internet; generally free programs, but not always. Carefully watch for "extra" items that will be installed and un-check or opt-out of them.

   You can check to see if you have any of these from [Fun] Web Products" installed:
       Control Panel > Add or Remove Programs, click on any that you find and click "Remove": Ask.com Bar, My Search Bar, MyWay Speed Bar, My Web Search Bar, Fun Web Products Easy Installer
       See:
           http://www.safer-networking.com/removemywebsearch.php
           PC Hell: My Web Search Removal Instructions
           http://helpint.mywebsearch.com/intlinfo/help/toolhelp.jhtml#q3 
   Also see: http://kb.mozillazine.org/Uninstalling_toolbars
   You MAY need to change a preference if when typing one or two words in the URL/Location/Address Bar sends you to some search page other than the one you expect. To reset your default URL/location bar search provider:
       Enter about:config in the address/URL bar and press the Enter key
       If you see a warning, accept it (Promise that you will be careful)
       Filter = keyword.URL
       Below the Filter, if keyword.URL is bold, right-click on keyword.URL and choose "Reset"
       Restart Firefox (File > Restart Firefox)
       See: http://kb.mozillazine.org/Keyword.url

Add code to userChrome.css below the @namespace line.

• http://kb.mozillazine.org/userChrome.css
• http://kb.mozillazine.org/Editing_configuration

You can use the ChromEdit Plus or Stylish extension to have easier access to the customization files.

• ChromEdit Plus: http://webdesigns.ms11.net/chromeditp.html
• Stylish: https://addons.mozilla.org/firefox/addon/stylish/

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* Set icon size on Bookmarks toolbar */
#personal-bookmarks .bookmark-item > .toolbarbutton-icon { height:16px!important; width:16px!important; }

/* Hide text on Bookmarks toolbar */
#personal-bookmarks .bookmark-item > .toolbarbutton-text { display:none!important; }