I had originally submitted bug 1590622 to complain about this, though it was ultimately repurposed to address another issue. Since this one already exists i won't make another, but i will mention the following:

• The lack of a height restriction causes the drop-down to, in some cases, span an almost comically large vertical space — i.e., from where-ever the form element is to the very top or bottom of the screen. Aside from being generally unsightly, this blocks out a lot of content on the page as well as any windows beneath the browser window, which can be a frustrating experience when you're trying to read something.

• It was mentioned that it might not be a common thing to have lots of saved log-ins for a single domain. Now that log-ins for sub-domains are included in the drop-down, i think it'll come up more often. It's especially problematic in the case of internal company networks, where you'll often have a million different Web applications under one domain (analytics.example.com, docker.example.com, git.example.com, mail.example.com, timekeeping.example.com, &c.). That was the scenario i ran into.

Sorry, I should have directed you to bug 1561647 instead, though they may both get solved together. I'll reply there.