Could you link to a page showing this problem?

When Multiprocess was rolled out around Firefox 50, the options of select controls reverted to pure unstyled system font appearance. Since then various style properties have been re-enabled. It's possible that this partial support is now causing strange results on some sites, but it would be easiest to look at specific cases.

Try the language dropdown on the top right of this page https://www.google.com/doodles

This drop down list inherits the CSS rules for the body element if I check this in the Inspector (right-click: Inspect Element) with a line-height: 1.2em.

body {
    font-family: 'Roboto',arial,sans-serif;
    font-weight: 300;
    line-height: 1.2em;
}

Yes it does inherit the CSS rules but the line-height element has no impact on a HTML dropdown. In FF56, the line-height was rendered by the browser at about 1.2em, in FF57 it appears to render it at about 2em. While this does not alter the functionality, it does make dropdowns bigger and thus less accessible to users with mobility impairments as well as less attractive when the content of the dropdown is narrow such as two characters e.g., ages or area codes on a form.

I suspect the increased line-height is intended for the minority of users with a touch screen (I am one of them) but should be linked to the "Density" setting in customize.