For your current problem, you can use https://addons.mozilla.org/en-US/firefox/addon/searchreset/ to revert these settings.

Unfortunately there isn't a real way to prevent this from happen. You can, when installing programs, make sure you read through every setting, and not install toolbars, search engines, etc. However, if the program is malicious they may not give you an options and will just install it anyway.

If you need to restore the about:home page to its default then reset the browser.startup.homepage_override.mstone pref via the right-click context menu to the default on the about:config page and close and restart Firefox.

• http://kb.mozillazine.org/about:config

1. Open the about:config page via the location bar
2. Type in the Filter bar: mstone
3. Right-click the browser.startup.homepage_override.mstone line and select: Reset
4. Close and restart Firefox

If this didn't help then delete the delete the chromeappsstore.sqlite file in the Firefox Profile Folder and repeat the above.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

Thank guys for the advice. I already removed both the offending search engine (Manage Search Engines... > Remove) and homepage (Options > General > Home page) - i didn't want to restore them to default, i use custom searche engines and restore tabs from previous session.

I guess i need to keep on the protection from "potentially unwanted applications" in my antivirus settings and live with occasional false positives.