Have you tried the builtin themes?

Designed before 4k monitors were made. I doubt if the specs will ever be updated.

https://developer.mozilla.org/en-US/Add-ons/Themes/Lightweight_themes

Image Requirements

   Dimensions should be 3000px wide × 200px high
   PNG or JPG file format
   Image must be no larger than 300 KB in file size

Image is aligned to the upper-right corner.

Thus the left side is 'missing' with 4,000 pixel wide screens. And with Quantum, users will never need that 200 px height. The days of 6 or 8 add-on toolbars is "history".

Mkll said

Have you tried the builtin themes?

Yeah but that's not the point.

the-edmeister said

Designed before 4k monitors were made. I doubt if the specs will ever be updated. Thus the left side is 'missing' with 4,000 pixel wide screens. And with Quantum, users will never need that 200 px height. The days of 6 or 8 add-on toolbars is "history".

Ah! So there are restrictions on the specs. That's unfortunate.

Thanks for the answers!

If it will not be fixed, they can simply delete ALL themes with background images, allowing only solid colours. 4K monitors are becoming more and more common, 5K monitors exist, even 8K monitors are on the way now, so NOT fixing this is the same as declaring firefox (or at least the mostpart of all themes in it) dead and out of its time.

The time may come when Mozilla needs to update the image size requirements or just stop supporting those lightweight themes, but I guess that time hasn't come yet.

I've discovered that one way to fix this is to go to about:config and change layout.css.devPixelsPerPx to 1.3 or higher. This scales up the entire browser, bigger ui, bigger text, bigger everything, so might not be a desired solution for everyone. If the browser could have a toggle "on/off/auto" or some sort of slider or possibly a simple plugin where users with screen-resolution higher than 3000 can set theme-scaling for only THAT background image and not the whole browser, it could work until some programmer behind firefox comes up with a more elegant solution to this.