Deleting My Login Email ID, and zmail not in Applications

Hello,

First, thank you for a great email client, it is a HUGE refreshing change from ad-ridden and bloated mainstream clients that get worse daily :)

I have two questions:
I am getting ready to shutdown/delete my yahoomail account (for reasons mentioned above and the reason I came to zoho), but I am not sure about what will happen. In other words, this yahoomail account is what is listed as My Login Email ID and also listed at the header of my Zoho mail page. Question is...If I delete yahoomail do I need another mail client to replace it for Login Email ID? Is this a requirement to keep Zoho working properly?

Second question:
I saw in Forums that we should be able to, in Firefox, access zmail via Tools>Options>Applications in order to make Zoho our default mail client, instead of the standard Outlook. However...zmail is neither offered in the available list, nor can I find it in Use Other option, or even on my computer anywhere at all using a search of all folders, etc. So, I'm not sure what to do here.

By the way I use Windows XP, and the latest Firefox, if that is relevant.

Thank you for any help in these areas, and....Happy New Year :D