Firefox, itself, makes no sounds, so it must be an add-on or something else. When do these sounds play? What was happening?

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Thank you for taking the time to reply. Much appreciated. The sounds started day before yesterday, just after the latest Firefox Update & Windows Update. After some reflection, I thought it was perhaps the Facebook Container Extension, and I removed it, but I heard another sound after that, so that was not the source.

I've been thinking about this a lot. Perhaps it is the "bell" on Twitter. I'm relatively new to Twitter, searched all over, could not find an explanation of how that bell works. Thought it might be like the "bell" on YouTube (silent notifications) but ???

I also have been wondering if it was Dell. (Have a relatively new computer, as well.)

I have not been hearing those sounds since early yesterday afternoon.

Again, I really appreciate your reply, and your time. Thanks so much.