When using Firefox on a multi-monitor setup where one of your screens (typically the laptop screen such as from Macbook) has very high DPI, the scaling of Firefox's content and menu items may not be appropriate when moved from one screen to the other.
By setting the layout.css.devPixelsPerPx to a value larger than 1 in
about:config, one can adjust the scaling of the browser. This value, however,
is not specific to the screen.
There are plugins such as AutoHiDPI that solve this issue but they are no longer supported in Firefox 57+.
Currently, setting layout.css.devPixelsPerPx to -1 seems to have fixed this
for me.