Hi, do you mean using the Apple Mail program, or within Thunderbird, to change the default send address?

If you want to be able to choose between Thunderbird and Apple Mail, you can set Firefox back to asking you which program to use instead of using Thunderbird automatically. Check out the steps in the following article, and select "Always ask": Change the program used to open email links.

If you mean within Thunderbird, a moderator can move your question over to the Thunderbird forum.

Thank you for your reply. Unfortunately, I followed the link and tried the option of changing the setting in the "preferences" menu of Firefox but I was not able to make it work.

So, I would appreciate it if a moderator would move my question to the Thunderbird forum.

ACTUALLY - I reread the linked article and WAS able to set the preferences after all. Thanks so much!