Hiding tab bar no longer works

### Describe the bug

`userChrome.hidden.tabbar = true` no longer hides the tab bar.

### Expected behavior

_No response_

### Screenshots

_No response_

### OS

Windows 10

### OS - Others

_No response_

### Firefox Version

119.0-7

### Distribution

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

### Theme

- [ ] Light
- [X] 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

I ran the project's included `user.js`, then deleted it, then changed the following settings in `about:config`:

```
userChrome.hidden.tabbar = true
userChrome.theme.fully_color = false
userChrome.theme.proton_chrome = false
```

### Additional context

I think the update to 119 broke it.

Reproduce the issue using the proton style on Windows 10 with Firefox 119.0-7 and the reported about:config values, especially userChrome.hidden.tabbar = true. Done means the tab bar is hidden again under this configuration; the issue does not mention a specific file or test to start from.

Written by the indexing model from the issue text.