Audio noise is usually caused by interference.

The most common issue is your Ethernet wire. Please ensure that the cable that connected to your PC's audio port is not close to or touching the cable(s) that plugin into your internet port on your computer. Since both of these wires are carrying electricity signals that send data, the audio wire's electric signal may be altered by other electric signals that pass close to the wire. Since Firefox and Thunderbird both use the internet, this could explain why you hear the noise only while using these programs.

For example, I often experience static and beeps on my audio speakers because my Ethernet cable is touching my audio cable.

I hope this will help. Let us know if it doesn't.

I separated the wires at the back of the tower case and the noise seems to have abated considerably, so thanks for the tip. I never would have imagined the Ethernet cable radiating that much noise, especially in that low frequency range. It would seem the induction of stray radiation was happening directly into the speaker leads, because that's the only audio connection back there, and turning off the audio on the desktop had no effect...the noise had to be entering "downstream" from the controls, at least. The speakers also have their own onboard audio power and there might just be a little bitty ground loop somewhere, but I don't think so...the noise was to high in frequency for a grounding problem hum.

Just goes to show, ya never know where the noise can come from. Thanks again for the tip.

Glad that I could help and provide some information for you. Thanks for contacting the Mozilla support.