URL bar rounded after upgrading to ESR 153

### Describe the bug

I am upgrading from ESR 140 to 153, and one of the things that breaks with photon-theme, is that the URL bar suddently became rounded. This doesn't match with the all the options in user.js I've set to disable rounding.

### Expected behavior

userChrome.rounding.square_button also squares the URL bar, as the items in the top bar are otherwise inconsistent. Alternatively, a separate setting.

### Screenshots

### OS

Windows 11

### OS - Others

_No response_

### Firefox Version

153.0.3

### 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)
- [x] 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

Light windows theme.

### `user.js` setup

(this is appended to the repository's user.js)
https://github.com/mid-kid/config/blob/952629dc85dbb45ca80769807c4458841683e155/zzz-firefox/user.js#L50

### Additional context

Applying this fixes it: https://github.com/black7375/Firefox-UI-Fix/issues/1139#issuecomment-3521601499
But I feel this fix is a bit of a workaround rather than a fix.
Result (changed the 2px in the fix to 0px):

Start in the photon-style SCSS rules for the URL bar and compare them with the workaround described in issue #1139. Verify the behavior with the reported Firefox 153 setup; done means userChrome.rounding.square_button makes the URL bar square without relying on the workaround.

Written by the indexing model from the issue text.