Why does Firefox interpret 100% zoom differently to other browsers?

Apparently this is some hot new feature Firefox introduced in 2013. I found a thread discussing it, where a lot of Firefox users seem to be unimpressed.

So it turns out there's a setting in Windows designed to stop text getting too small on high-resolution devices. You can find it by searching for "text size" in control panel.

On my machine, it's set by default to scale text up to 125%:

Apparently, Firefox has decided that it's going to quietly take that setting, and pretend that 100% equals whatever that setting is. So for me, in Firefox-land, 100% = 125%.

It applies this to the Firefox UI as well as all web content (which is why my Firefox buttons look blurry).

It sounds like there's no simple way to turn this off - the thread linked above describes some complex steps, then loads of people comment saying they don't work.

Setting the Windows sizer to 100% makes them more similar (slightly smaller in Firefox, oddly), but it also makes everything painfully small, so it doesn't really solve the problem.

As previous answer stated, it was introduced as a feature. But it CAN be overridden. I found this post explaining the 'feature' and how to revert the behavior.

if you want to set the text size/pixel density in firefox different from that of your system's settings like it was handled in prior versions, enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named layout.css.devPixelsPerPx. double-click it and change its value to 1.0 (or any other zoom factor that fits your purpose; -1.0 is the default value and will adhere to the system settings).

https://support.mozilla.org/en-US/questions/963036