The green plus means that the website offers a search engine as you can see if you click the search engine icon.

The browser.search.showOneOffbuttons pref is no longer supported in Firefox 43+, so modifying this pref won't have effect. You can reset the pref via the right-click context menu to the default value.

You can look at the Classic Theme Restorer to restore the previously used search bar. You will find this feature in the CTR Options/Preferences.

• CTR Options > General UI (1) -> Old search (experimental)
• https://addons.mozilla.org/firefox/addon/classicthemerestorer/

You can use a custom style rule to override the special icon and show the standard one. You can apply custom style rules to Firefox's interface using either the Stylish extension or a userChrome.css file. For example (tested on Windows 7):

.searchbar-search-button[addengines="true"] {
  list-style-image: url("chrome://browser/skin/search-indicator.png");
  -moz-image-region: rect(0, 20px, 20px, 0);
  margin-top: 1px;
  margin-bottom: 1px;
  -moz-margin-start: 4px;
  width: 20px;
}
.searchbar-search-button:hover {
  -moz-image-region: rect(0, 40px, 20px, 20px);
}
.searchbar-search-button:hover:active {
  -moz-image-region: rect(0, 60px, 20px, 40px);
}
@media (min-resolution: 1.1dppx) {
  .searchbar-search-button[addengines="true"] {
    list-style-image: url("chrome://browser/skin/search-indicator@2x.png");
    -moz-image-region: rect(0, 40px, 40px, 0);
  }
  .searchbar-search-button:hover {
    -moz-image-region: rect(0, 80px, 40px, 40px);
  }
  .searchbar-search-button:hover:active {
    -moz-image-region: rect(0, 120px, 40px, 80px);
  }
}

I just incorporated that into my custom style rule here, if you're interested in a list with names rather than only icons:

https://userstyles.org/styles/122214/firefox-search-bar-show-engine-names-firefox-43