That is a reference to a setting on the General page of Settings/Options (see screenshot).

Thanks for your reply and screenshot. Unfortunately, that didn't work because my Mailto Setting in Firefox is set to Mail (default). As I test, I went into Windows Settings/Apps/ Default Apps and changed the Mail default from Windows 10 to Outlook. Now the message is a bit more specific (see the screenshot).

So, what application will I be adding to my system if I say yes? And what does it do? And if I don't want how does a user turn off this message????

So, what application will I be adding to my system if I say yes? And what does it do? And if I don't want how does a user turn off this message????

Adding the "application" actually adds a couple of lines to a configuration file named handlers.json which is consulted when Firefox needs to know how to open a link with a protocol it doesn't handle itself, such as mailto: or zoommtg:. I have attached a screenshot showing where that goes and what it looks like for the one offered by Outlook.

The bar stops appearing if the offered mailto handler is already saved. However, if you decide you do not want to save it -- because you never ever want to open a mailto link in Firefox directly into Outlook online -- I am not aware of a way to prevent the bar from appearing. With some bars, using the little X on the right will be "remembered" for future visits, but I assume you already tied that?

You are correct, the "X" on the bar removes the message. But then it shows up the next day when I start up my system. So you have talked me into it. I'll click on the button. Also marking this as solved.

Hey guys! I may have had the same issue and found a really neat and helpful answer at Superuser .com to attempt in about:config :

"This recently just happened to me, after over 15 years of using it. Changing network.protocol-handler.external.mailto to false seems to have worked for me. – Asinine. Sep 21 '20 at 21:12" --> https://superuser.com/questions/363827/how-can-i-disable-add-application-for-mailto-links-bar-in-firefox

WARNING: Changing preferences through this interface not officially supported Hidden settings edited using the about:config tool are explicitly not supported, which means that Mozilla makes no guarantees they will be supported in the future, or that Mozilla will fix them if they break. Mozilla does not test these preferences, and will not in the future. That includes security and performance testing which these preferences may affect.

Here is hope it helps! Cheers!

[Warning added by moderator.]