One staff person's new iMac (4K, 10.13.6) has been scaled to a larger text size to make it more readable for her (one size up in the "Scaled" display settings). On her secondary monitor (default, not scaled), and only in Firefox (61.01), occasionally the font size in the browser will be extremely large.
She can make it smaller by dragging the Firefox window to the iMac and back again to the second monitor, but if she changes tabs or goes to another page in the same tab, the size may increase again. She also notes that frequently, line items in drop-down boxes will not all display in Firefox - the missing ones will appear if she drags the window to the iMac. (Not sure if this is a scaling/font size issue or something else.)
In System Preferences > Mission Control, the setting "Displays have separate spaces" is checked. Secondary display is an Acer 17" monitor connected via a DVI-T3 adapter. Not sure what other info may be relevant.
She needs to be able to use both Chrome and Firefox, and would like to keep the larger scale on the iMac if possible. Her workflow would suffer if she has to rearrange the way she uses her browsers and screens.
Has anyone run across this issue before, and is there a way to fix it?