Could you please explain this Extension you have installed ? Search Web - VLC App 1.0.17.1031 ({349526f1-bac0-459d-b2d0-ee5a27b54ba2})

VLC is a video player , I would suggest removing this extension. Please give your self a scan with this : https://www.malwarebytes.com/mac/

Or it could just be a file glitch that a uninstall and deleting Firefox Folders and a fresh in stall with a Full Version Installer could fix. I do not know what the Folders are called on a Mac nor where they are. I will provide the windows info for you to work with : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Note: your Firefox Profile is saved. But you should make a back up before you do :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 58.0.2 with a Full Version Installer

• https://www.mozilla.org/firefox/all/

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

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 other file found here is suspicious and you can check the content of the file in a text editor.

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.