Tab look regression in versions 126.* and above (after 125.0.3)

### Describe the bug

This is version 128.0.
Firefox-UI-Fix version: latest


### Expected behavior

That's how it looked in 125.0.3
Firefox-UI-Fix version: [v8.5.1](https://github.com/black7375/Firefox-UI-Fix/releases/tag/v8.5.1) (I think)


### Screenshots

Above.

The blue border on the tabs on the sides is now gone, and there's also no border at the bottom of the tabs, which there should be, because the gray border between tabs still works.

### OS

Linux

### OS - Others

_No response_

### Firefox Version

128.0

### 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

Marwaita (old version)

### `user.js` setup

Let me just upload the whole chrome dir. This is the working version which I used in 125.0.3.

[chrome.zip](https://github.com/user-attachments/files/16161573/chrome.zip)

[user.js.zip](https://github.com/user-attachments/files/16161881/user.js.zip)

### Additional context

_No response_

Reproduce the report on Firefox 128 with the photon-style distribution, System Default theme, and the supplied chrome.zip and user.js.zip setup. Compare the tab appearance with Firefox 125.0.3 and v8.5.1, focusing on the missing blue side and bottom borders. Done means the expected tab borders are restored without breaking the existing gray separator.

Written by the indexing model from the issue text.