http://www.clamxav.com/ free malware scanner for Mac OS X https://discussions.apple.com/docs/DOC-3291

You can also try https://www.malwarebytes.org/antimalware/mac/index.html

You may be able to change your Firefox search engine, and remove the option to use chumsearch. See

• Add or remove a search engine in Firefox_remove-search-engines_2
• Change your default search settings in Firefox

Did you find chumsearch or some other unexpected search engine ? and were you able to remove it, and set Firefox back to your chosen search engine ?

Your Mac should have an automated Malware Removal Tool (MRT). Check that is on

To ensure that MRT will run when that database is updated, open the App Store pane in System Preferences and check the box marked

Install system data files and security updates 

if it's not already checked.

That of course only works if Apple Mac has registered whatever you have as malware (Rather than a legitimate search enhancement or adware).

• Did that help ?

Such problems are more often seen on Windows PCs. You may find this Malwarebytes software will help find the culprit

• https://www.malwarebytes.org/antimalware/mac/

Your troubleshooting information may help us. You should also check the shortcuts Firefox uses, although I am not sure how to do that on a Mac.

Please post back to say how you get on.

Troubleshooting Information

To find the correct solution to your problem, we need some more non-personal information from you. Please do the following:

1. Use ONE of these methods to open the Firefox Troubleshooting Information page:
   • Click the menu button , click on help and select Troubleshooting Information.
   • Type about:support into the Firefox address bar and press the enter key.
2. At the top of the Troubleshooting Information page that comes up, you should see a button that says "Copy text to clipboard". Click it.
3. Now, go back to your forum post, right-click in the reply box and select Paste from the context menu (or else click inside the reply box and press the Ctrl+V keys) to paste all the information you copied into the forum post.

If you need further information about the Troubleshooting information page, please read the article Use the Troubleshooting Information page to help fix Firefox issues.

Try to delete the search.json file and possible search-metadata.json and search.sqlite files in the Firefox profile folder to reset the search engines to the default.

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

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Firefox will rebuild the search.json file from the default built-in search engines that come with Firefox and the searchplugins folder in the Firefox profile folder.