prevent apps from changing status bar color never cared much for this one way or another, until Firefox last update changed the status bar to white, of all colours. 

now it really bothers me. my Firefox theme have darker navbar, like the branding on the desktop. they used white navbar on android now for the misguided reason of stealing chrome's look. I think the older darker navbar differentiates the browser UI from the page, and thats good from a usability point of view.

the correct fix for this will be for themes to select what Firefox paint the status bar too, so it doesn't look too weird with status bar in white with address bar in black.

but until then, is there any option to disable apps setting the status bar colour? (doesn't matter if per app or globally. consistency is good UX)
Yes, there are apps that allow you to override the status bar completely. I had the same issue with Firefox's horrid new UI choice and was able to solve it with the Status app. I can't say if it's the best app for the job, it's just the first one I tried. So far it seems fine although it took some time to get it set up how I wanted. If that doesn't do it for you there are plenty of other apps on the store that seem to do the same thing.