This could be an issue caused by AdAware or Web Companion from Lavasoft.

You can look for a file named dsengine.js in these locations. You should only find channel-prefs.js in the "defaults\pref" location. Any file found here apart from channel-prefs.js is suspicious. You can check the content of the file in a text editor (use open with and do not double-click the file).

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

You can look for a file named dsengine.cfg in the main Firefox program folder.

C:\Program Files\Mozilla Firefox\ C:\Program Files (x86)\Mozilla Firefox\

Delete the dsengine.js and dsengine.cfg files when present.

Please go through your list of installed programs and look for something you do not know. It may have come in with something you installed and did not tell you about the time this started.

• https://www.malwarebytes.com/mac/
• https://www.kaspersky.com/free-mac-virus-scan
• https://www.trendmicro.com/en_ca/forHome/products/housecall.html
• https://www.bitdefender.com/solutions/virus-scanner-for-mac.html

Always use custom install when available and watch closely as to what you click on and the wording. Even then it can still get through.

Please let us know if this solved your issue or if need further assistance

^: Note that the OP in on Mac, so what you posted above doesn't apply.

Do you otherwise have issues with keeping preferences?

Changing the default search engine can either be a problem with prefs that somehow get reset or a private browsing with search.json.mozlz4 (this file is compressed).