Go to Tools | Options | Applications and select mailto. Click on whatever it has to the right. Choose "Use Other ...". Then, if Zimbra is not listed, choose Browse, and navigate to the folder where Zimbra is installed, and choose the main program file. If you don't know what that is, go the shortcut you use to start it, right-click on it, choose Properties, and you can find out where it is under Shortcut | Target.

I think Zimbra is from comcast. I can't find anything for Zimbra or comcast webmail on my computer. Thanks

Oh. Well, don't click on the button, but just go to the Zimbra web mail page when you want to compose an email instead. It is uncommon, but it has been done, that a web mail can be installed in a way that clicking a mailto: link will start creating a message in a web mail email.

Just right-click on the link you want to reply to, copy the link location/address, open up your browser, go to the Zimbra site, paste the link into the "Send to" field, remove the "mailto:" part of the address, write and send yer meg.

See this support article - https://support.mozilla.org/en-US/kb/change-program-used-open-email-links?esab=a&s=mailto&r=0&as=s