Bugzilla

It looks like the problem here is actually that the close button (`<image class="tab-close-button close-icon">`) **gets wider** when you hover the tab title.  (The close-button's bounds aren't directly visible, so you can only see it getting wider via this indirect effect on the neighbor label).

Specifically, this is the problematic rule that's causing this bug:
```
.tabbrowser-tab:not(:hover) > .tab-stack > .tab-content > .tab-close-button {
    padding-inline-start: 0;
    width: 19px;
```

When the tab-title is not hovered, this CSS rule applies, and the close button is 19px wide.
When the tab-title **is** hovered, this CSS rule stops applying, and another declaration takes over to set `width: 24px`, which is 5px wider. This results in the label getting 5px skinnier, since they have to share space.

It looks like the problem here is actually that the close button (`<image class="tab-close-button close-icon">`) **gets wider** when you hover the tab title.  (The close-button's bounds aren't directly visible, so you can only see it getting wider via this indirect effect on the neighbor label).

Specifically, this is the problematic rule that's causing this bug:
```css
.tabbrowser-tab:not(:hover) > .tab-stack > .tab-content > .tab-close-button {
    padding-inline-start: 0;
    width: 19px;
```

When the tab-title is not hovered, this CSS rule applies, and the close button is 19px wide.
When the tab-title **is** hovered, this CSS rule stops applying, and another declaration takes over to set `width: 24px`, which is 5px wider. This results in the label getting 5px skinnier, since they have to share space.