Have seen this problem mentioned on the internet without a definitive solution.

My Firefox browser was upgraded to the above mentioned version without any advisement and contrary to my upgrade options, so when the loss of audio in Firefox occurred I was at a loss.

After considerable checking the internet for possible fixes and exploring options in Mint I arrived at a simple solution, at least on this desktop system.

Menu - Control Center - Sound - (launch an audio feature in Firefox such as a youtube video) - Applications - Check that the AudioIpc server audio level control is not muted or turned down too low.

Regards, lax

Thanks Lax. Do note that to mark a thread as "Solved", you use the "Thread Tools" menu at the top of the thread.