when you leave your game in the background, whatever type of powerful gaming pc you have, it will always change to 1440p unless or until you switch the game off.

That would stand to reason - unfortunately this problem occurs after I have closed the game and is only resolved on closing and relaunching Firefox.

There maybe something useful in this search, not sure. Also check the page bottom suggestions : https://www.google.ca/search?q=firefox+%2B4k+%2Bdpi&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&dcr=0&ei=HC8-WvXhCe-E0gKBqIGgAg

Please let us know if this solved your issue or if need further assistance.

I've come across similar in my searches before - I would agree that setting DPI mode manually would seem like an agreeable solution, though doing this via page zoom would still leave tabs and menus undersized. I haven't tried to manipulate any of the compatibility menu settings yet and this would seem like it might plausibly solve the problem. Unfortunately I am away from my PC until after Christmas now but will feed back on progress once I've tried.

Having thought about the problem some more, I think the triggering application was actually Steam, running in big picture (i.e. fullscreen) mode at 4k.

My problem was resolved by setting "override high DPI scaling behaviour" in the compatibility menu of firefox to "Application".

Thanks for the suggestions.

I am glad to hear that your problem has been resolved. If you haven't already, *Please select the answer that solves the problem*YOURS. This will help other users with similar problems find the solution more easily. Thank you for contacting Mozilla Support.