The default search engines are supplied as a part of Firefox, so the easiest way is to reinstall. (Other ways such as trying to download them separately would be kind of challenging.) Here's how I suggest doing it:

Clean Reinstall

We use this name, but it's not about removing your data/settings, it's about making sure the program files are clean. 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 36.0.4 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox.

(3) Rename the program folder

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\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

Actually I figured out where the folder for the default search engines was and replaced it using another copy. The oldfirefox folder I had from the reset was empty.

This used to be a trusted piece of freeware.