The desktop Firefox any version for Windows, Mac, or Linux does not come with malware. Your issue may actually be related to something outside of Firefox, say from installing AdAware or Web Companion from Lavasoft. https://support.mozilla.org/en-US/forums/contributors/712839

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 in most of the world (Yandex and Baidu in some countries). Possibly you installed the Lavasoft program '''Ad-Aware Web Companion'''? That program drops a script file that overrides your Firefox default search engine every time you start the browser. It creates "Bing Search Engine" (the built-in "Bing" search plugin is just called "Bing"). To remove this rare <b>program folder infection</b> that malware cleaners tend to miss, 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. ---- If that isn't the problem, then I suggest the usual clean-up procedures. <p>(1) Open the Windows <strong>Control Panel</strong>, Uninstall a Program.</p> <p>After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help in smoking out undisclosed bundle items that snuck in with some software you agreed to install. <em>Be suspicious of everything you do not recognize/remember, as malware often uses important or innocent sounding names to discourage you from removing it.</em></p> <p>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.</p> <p>(2) Open Firefox's <strong>Add-ons page</strong> using either:</p> <ul> <li>Ctrl+Shift+a (Mac: Command+Shift+a)</li> <li>"3-bar" menu button (or Tools menu) &gt; Add-ons</li> <li>in the Windows "Run" dialog, type or paste<br>firefox.exe "about:addons"</li> </ul> <p>In the left column, click <strong>Extensions</strong>. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions. <em>All extensions are optional; the troublemaker might not be obvious from its name.</em></p> <p>Any improvement?</p> <p>(3) You can search for remaining issues with the <strong>scanning/cleaning tools</strong> 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.</p> Hopefully that will flush anything on your system and in your Firefox that could be hijacking your search preferences.