blank images on new tab for firefox version 155.0.1 - userContent.newTab.full_icon = false is a workaround

### Describe the bug

Since the update the new tab full_icon setting is different so when i disable it in the userjs it fixes the problem. Not sure if it is the calc(var(--size-item-large) * ...) since my experience is very limited. I also want to note the icon container is now div.icon-stack, not div.icon-wrapper after inspect element so i am not sure how to fix except for the workaround

### Expected behavior

_No response_

### Screenshots

### OS

Windows 11

### OS - Others

_No response_

### Firefox Version

155.0.1

### Distribution

- [ ] [Original Lepton](https://github.com/black7375/Firefox-UI-Fix)
- [x] [Lepton's photon style](https://github.com/black7375/Firefox-UI-Fix/tree/photon-style)
- [ ] [Lepton's proton style](https://github.com/black7375/Firefox-UI-Fix/tree/proton-style)

### Theme

- [ ] Light
- [ ] Dark
- [ ] Alpenglow (System: Light)
- [ ] Alpenglow (System: Dark)
- [ ] System Default
- [ ] Others: (e.g. [Photon Colors](https://addons.mozilla.org/en-US/firefox/addon/photon-colors/), [Google Chrome Dark](https://addons.mozilla.org/en-US/firefox/addon/google-chrome-dark/), [Fairytale Of Nature](https://addons.mozilla.org/en-US/firefox/addon/fairytale-of-nature/))

### Theme - More Info

_No response_

### `user.js` setup

[user.js](https://github.com/user-attachments/files/31860806/user.js)

user.js setup

```javascript
// Your settings
```

### Additional context

_No response_

Start by comparing the photon-style new-tab image and icon styling around the reported div.icon-stack change, using userContent.newTab.full_icon as the reproducible setting on Firefox 155.0.1. Verify the result with full_icon enabled; done means new-tab images render without requiring the workaround.

Written by the indexing model from the issue text.