It is not that. All my other program windows are scaled correctly when I move them to the second display. It is only Firefox that is doing this.

I’ve already used the about:config to set the main display to 2. If I do it again once I’ve dragged the window over to the second display then it rescales on my main monitor too which I don’t want. I just want the window to be the same scale on BOTH monitors.

I'm on a Macbook Pro from 2012 running 10.13.6 and here are my settings.

Displays: Color LCD:

 Display Type:	Built-In Retina LCD
 Resolution:	2880 x 1800 Retina
 Framebuffer Depth:	24-Bit Color (ARGB8888)
 Main Display:	Yes
 Mirror:	Off
 Online:	Yes
 Rotation:	Supported
 Automatically Adjust Brightness:	No

HP Z27n:

 Resolution:	2560 x 1440 (QHD/WQHD - Wide Quad High Definition)
 UI Looks like:	2560 x 1440 @ 59 Hz
 Framebuffer Depth:	24-Bit Color (ARGB8888)
 Display Serial Number:	CNK7240MFG  
 Mirror:	Off
 Online:	Yes
 Rotation:	Supported
 Automatically Adjust Brightness:	No

What are your settings on your external monitor?

Here are my settings, they are both set to the same resolution.