[FF] How to add favicon.ico on your POP/IMAP account.

Forums:

After you type... about:config in the location bar and press Enter

...you can type:  xnotifier   ...to show all the lines for xnotifier accounts
...or you can type: IMAP  ...to show all lines containing the 'word' IMAP

Look your IMAP account for example
extensions.xnotifier.accounts.[imap#user.name @ domain.com|imap.mail.domain.com].inboxOnly
[Right-click] Copy Name
[Right-click] New -> String
[Paste and backward delete] inboxOnly
[Type] icon
[Click] OK

Next pop up menu look alike this

extensions.xnotifier.accounts.[imap#user.name @ domain.com|imap.mail.domain.com].icon

You look at your website with /favicon.ico to test,
if it is work for example

HTTP:// www. domain .com/favicon.ico

Some of them are complex alike
http://mb.softbank.jp/mb/shared/img/frm2/favicon.ico
(Actually, it works that is mine at Softbank in Japan.)

Another example that works too.

http://www.icloud.com/favicon.ico
https://mail.google.com/favicon.ico (as of June 29, 2017)
http://s.uicdn.com/mailint/5.856.0/assets/favicon.ico (as of June 29, 2017) (Mail.com)
https://www.yahoo.com/favicon.ico (as of July 12, 2017, and thanks to RpD)

It always can be found in the HTML header:
Just open the HTML source (Ctrl+U) and search for a favicon. (Thank You jeroen for suggesting.)

If you successful favicon.ico on website and copy whole URL, paste, and [Click] OK

FYI:
POP account it works the same way as I explain above about IMAP account as an example.

IF:
One account POP and one account IMAP, you do not need to worry about favicon logo.
But, you have two or more account on POP and IMAP, it will show up "not correct" favicon logo.

TO SOLVE:
[Right-click] X-notifier
[click] Options
[Click] Accounts (tabs)
[Look host with IMAP account and click]
(Alias) [Type] IMAP/(fill-in as you wish)
and
[Look host with POP account and click]
(Alias) [Type] POP/(fill-in as you wish)

FOR EXAMPLE:
IMAP/work icloud.com
IMAP/home icloud.com
IMAP/Mac mail.com
IMAP/iPad mail.com

POP/work hotmail.com
POP/home hotmail.com
POP/PC mail.com
POP/Anoirid mail.com

AND:
Enjoy your favicon.ico with your POP/IMAP account(s) !

Thanks to RpD for corrections.