Can you post a screenshot of the warning?

Based on this thread, your computer probably has malware that is making this happen. Try running a legitimate antivirus and see if it finds anything.

If you're looking for some free antivirus scanners, our support article lists tools other Firefox users have found helpful: Troubleshoot Firefox issues caused by malware.

The updater.exe program is the Firefox updater that you should allow to run. If the program name is really update.exe then you should check its location.

I'll try. Thanks.

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