After removing all passwords under three dots/kebab menu in the top corner of the page "about:logins", i wanted to import passwords from a Proton Pass to see how it works. Yet there is no import menu entry. After researching, I have found a solution that I want to share:

go to "about:config" page, and search for "signon.management.page.fileImport.enabled". Set it to true by double clicking on "false". Reload "about:logins" page to see the import menu entry under three dots corner menu.