Firefox is designed to allow you to pick your default search engine and have it remain unchanged -- at least until a new company signs up with Mozilla as the default search provider, but that doesn't happen very often. The current default actually is Google.

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 that Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage.

Anything on the list that seems to be related to searching or to Yahoo? Anything you don't remember installing or why you have it? If in doubt, disable or remove.

Any improvement?

I've tried removing anything from my add-ons that looks like it might be doing that, but Yahoo is still reappearing every time I restart Firefox.

I have Avast installed on my computer and I know it's screws things up, could that be what's adding Yahoo to my search? If so, does anyone know how to stop it?

I'm not aware of Avast causing a problem with choosing your preferred default search engine. However, other external software could be a factor.

Here are suggested next steps. I know it seems long, but it's really not that bad.

(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 known badware 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.

(3) A program folder infection is unusual, but you can 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. Any other files are suspicious. Remove them to a neutral location for further analysis at your leisure.

Hu, that was strange. I followed all steps and when I tried the last I found this file in default folder

• dsengine.js

After I moved it to another folder and restarted Firefox I was happy to see Yahoo was finally gone for good.

Thanks for the help! ^_^

GreenTea13 said

I followed all steps and when I tried the last I found this file in default folder

• dsengine.js

The two anti spyware/virus/etc programs I have are Avast and SUPERAntiSpyware (both Free Edition), so maybe one of those did it?

I doubt it's Avast, or this would be an epidemic by now...