Aside from the details discussed in bug 1561647 reducing the visible items before scrolling, it would probably help alot to also consider having a way to diferentiate either by contrast (e.g. different backround) or maybe a separator betwen what comes from "this website" and the rest of subdomain password manager entries.

Currently we diferentiate the above by having "this website" and the origin for the rest. (see Firefox dropdown from https://bug1601558.bmoattachments.org/attachment.cgi?id=9121474). IMO, one problem with it is also the perceived wall of text, which require actual reading of the origin, so either by changing the font formatting (e.g italic for the the subdomains) or e.g. adding a backround to isolate the "from this website" from the subdomains seems like a good idea for me.

as i mentioned in #1611722, the "perceived wall of text" is imperceptible until careful attention is put.

For some sites that are used very often, the lasted used, or a combination of most used+recently, should float up and given prominence.

Drive-by UX suggestion:

• originally show only passwords "from this website", with a single item "show passwords from related sites" (if any)
• if the user clicks "show passwords from related sites" then expand the list as it is today
• if the user picks one of those, show the usual "Save this password for this site?" prompt and create a new login entry.

Advantage is simplicity: practically no new UI elements (one string), and no tweaks to password storage. Won't automatically pollute the list for next time if the user chose incorrectly.