Manually Refresh POP Mail Fetching in Gmail
This script adds "Check Mail" button on the left top of the screen.
Only works with stylish addon and you must have download this style
https://userstyles.org/styles/121818/check-mail-button-for-pop3-accounts-in-gmail
and after that, install gmail check.

Note: Only works with one POP mail account.
If you have two or more, button only check last pop account.