Firefox normally uses your operating system's default mail program to send an email message. This article explains how you can change the program that is launched when you click on:

* The Email Link… menu item in the Firefox Page actions (3-dot) menu. 
* The Firefox Email linkbutton or menu item (if you customized Firefox controls, buttons and toolbars). 
* The Email Image… (right-click) context menu item for an image on a web page. 
* A "mailto" or email link on a web page (such as email@example.com). 

https://support.mozilla.org/en-US/kb/change-program-used-open-email-links

My main problem is that when a website offers me an email send link and it proposes to send my email (with my correct email address) via 'Edge' or some MS. If/when I get reply(s) I have to deliberately go to my own yahoo mail tab, activate & choose to deal with it there. otherwise it would default to an MS email program.

When I choose to send an email, I click my yahoo tab in my Firefox browser, no problem there to send or receive a reply. I think it's only when I click on the proposed recipients email address from their website?

I never usually have any problem choosing sending from my yahoo tab in the Firefox browser.