I noticed that when you start to load a Unity WebGL game in Chrome, if the window loses focus (you open a new tab, open a new window, or switch to a different app), the game loading pauses.
This doesn’t happen on Firefox. If you start to load a Unity WebGL game in Firefox, open a new tab, and come back, the game will be loaded.
Is this something that we can change or have any control over?
There is no control over what a tab does when it looses focus. That is up to the browsers.
Giving this a quick test it also loads in an inactive tab in Chrome it just takes longer. If you compare active tab load times for your game in FF and Chrome you can see the difference and the same difference applies to inactive tabs and probably even more so since inactive tabs get less priority from the browsers.