I'm using Firefox on a Mac. Whenever I switch from using a different application back to Firefox it changes which window is at the front.

For example, sometimes I am writing something in Firefox and need to reference a file that is open in Preview. I'll switch to Preview to look what I need to reference, and then when I press command + tab to switch back to Firefox, rather than taking me to the window that I most recently had open it takes me to another window that was in the background. It seems like it only started recently but I didn't make any obvious changes before it started happening. Any reason why this might be happening and how to change it?