# Question about image fetching in Responsive Design mode

**URL:** https://discourse.mozilla.org/t/question-about-image-fetching-in-responsive-design-mode/146082
**Category:** Developer Tools
**Created:** [September 27, 2025, 12:25am UTC](https://discourse.mozilla.org/t/question-about-image-fetching-in-responsive-design-mode/146082 "2025-09-27T00:25:01Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![Dongyeong\_Chon](https://sea1.discourse-cdn.com/flex001/user_avatar/discourse.mozilla.org/dongyeong_chon/32/74967_2.png) [@Dongyeong\_Chon](https://discourse.mozilla.org/u/Dongyeong_Chon)
#### Post date: [September 27, 2025, 12:25am UTC](https://discourse.mozilla.org/t/question-about-image-fetching-in-responsive-design-mode/146082/1 "2025-09-27T00:25:01Z")

</div>

Hi, fellow developers.  
I have a question about Firefox’s image fetching behavior when the Responsive Design mode is on.

TL;DR) When the Responsive Design mode is on, why does Firefox fetches images with srcset attribute twice?

I was testing with this website ([Responsive images &nbsp;|&nbsp; web.dev](https://web.dev/learn/design/responsive-images)) ‘s first image.  
The image has `srcset` and `sizes` attributes like this below:

```auto
// image URLs are simplified
srcset="image_36.png 36w,
        image_48.png 48w,
        image_72.png 72w,
        image_96.png 96w,
        image_480.png 480w,
        image_720.png 720w,
        image_856.png 856w,
        image_960.png 960w,
        image_1440.png 1440w,
        image_1920.png 1920w,
        image_2880.png 2880w"

sizes=”(max-width: 840px) 100vw, 856px”

```

I set the RDM to iPhone 11 Pro IOS 14.6, which is `width: 375px` and `PDR: 3`.

I thought that the browser should only fetch `image_1440.png`.

```auto
1. (max-width: 840px) condition is true, so display size will be 100vw.
2. 100vw of 375px is 375px.
3. Required physical pixel size is 375px * DPR = 375px * 3 = 1225px.
4. The closest image to 1225px is image_1440.png

```

However, when I check the network tab, the browser actually fetches the image twice ( not only this image, but whole images on this page are fetched twice).  
The first is `image_2880.png` and the second is `image_1440.png`.

Can someone help me understand what’s going on here?