That's suspicious!

One possible culprit would be an extension. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions.

Then cast a critical eye over the list on the right side. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove).

Any improvement?

A less common method of infiltration is a program folder infection. Check in these locations (varies for 32-bit / 64-bit):

• C:\Program Files\Mozilla Firefox\defaults\pref
• C:\Program Files (x86)\Mozilla Firefox\defaults\pref

A file named channel-prefs.js is normal. A file named dsengine.js or any other file is suspicious. Remove any such files to a neutral location for further analysis at your leisure, or bury them in the Recycle Bin.

I also suggest the usual clean-up procedures.

(1) Open the Windows Control Panel, Uninstall a Program.

After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help surface undisclosed bundle items that snuck in with some software you agreed to install. Be suspicious of everything you do not recognize/remember, as malware often uses important or innocent sounding names to discourage you from removing it.

Take out as much trash as possible here. If you're not sure, feel free to post program names or a screenshot of the list.

(2) You can search for remaining issues with the scanning/cleaning tools listed in this support article: Troubleshoot Firefox issues caused by malware. 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.

Hopefully that will flush anything on your system and in your Firefox that could be hijacking your search preferences.

Hello, I removed the new search engine again and disabled all my addons except AdBlocker. Closed Firefox, and shut down my PC. The intrusive search engine did not appear again after I started Firefox. I think you are right about it being a problematic extension that came with this little 'gift'. Most of the extensions I use are widely used (I don't have anything that is under 10.000 users) and well liked - I will try to investigate which one it is. Thank you very much for all the help.

Hi, if you do find out which one it is please go to the https://addons.mozilla.org/firefox/ page of the Extension and there is a abuse link on the page. Please post/report your findings.