Bugzilla

I'm not sure what you mean by the first point, but you're correct that:
- No behavior change whatsoever for horizontal tabs
- No behavior change for when a new tab is added via the tab right-click menu
- Behavior change only for when a new tab is opened via the vertical tabs new tab button, and then yes it would be inserted at the start of the list.

After a first pass at looking into this, I think it might be a matter of adding a condition [here](https://searchfox.org/mozilla-central/rev/261005fcc4d6f8b64189946958211259fb45e9e1/browser/components/tabbrowser/content/tabbrowser.js#3628)
```
 // Ensure index is within bounds.
      if (pinned) {
        index = Math.max(index, 0);
        index = Math.min(index, this._numPinnedTabs);
      } else {
        index = Math.max(index, this._numPinnedTabs);
        index = Math.min(index, this.tabs.length);
        index = this.tabContainer.verticalMode && !openerTab ? 0 : index;
      }
```
But we'll need good test coverage to be sure.

I'm not sure what you mean by the first point, but you're correct that:
- No behavior change whatsoever for horizontal tabs
- No behavior change for when a new tab is added via the tab right-click menu
- Behavior change only for when a new tab is opened via the vertical tabs new tab button, and then yes it would be inserted at the start of the list.

After a first pass at looking into this, I think it might be a matter of adding a condition in [_insertTabAtIndex](https://searchfox.org/mozilla-central/rev/261005fcc4d6f8b64189946958211259fb45e9e1/browser/components/tabbrowser/content/tabbrowser.js#3628).

So something like `index = this.tabContainer.verticalMode && !openerTab ? 0 : index` in the `else` block.
```
 // Ensure index is within bounds.
      if (pinned) {
        index = Math.max(index, 0);
        index = Math.min(index, this._numPinnedTabs);
      } else {
        index = Math.max(index, this._numPinnedTabs);
        index = Math.min(index, this.tabs.length);
        index = this.tabContainer.verticalMode && !openerTab ? 0 : index;
      }
```
But we'll need good test coverage to be sure.

I'm not sure what you mean by the first point, but you're correct that:
- No behavior change whatsoever for horizontal tabs
- No behavior change for when a new tab is added via the tab right-click menu for vertical tabs.
- Behavior change only for when a new tab is opened via the vertical tabs new tab button, and then yes it would be inserted at the start of the list.

After a first pass at looking into this, I think it might be a matter of adding a condition in [_insertTabAtIndex](https://searchfox.org/mozilla-central/rev/261005fcc4d6f8b64189946958211259fb45e9e1/browser/components/tabbrowser/content/tabbrowser.js#3628).

So something like `index = this.tabContainer.verticalMode && !openerTab ? 0 : index` in the `else` block.
```
 // Ensure index is within bounds.
      if (pinned) {
        index = Math.max(index, 0);
        index = Math.min(index, this._numPinnedTabs);
      } else {
        index = Math.max(index, this._numPinnedTabs);
        index = Math.min(index, this.tabs.length);
        index = this.tabContainer.verticalMode && !openerTab ? 0 : index;
      }
```
But we'll need good test coverage to be sure.

I'm not sure what you mean by the first point, but you're correct that:
- No behavior change whatsoever for horizontal tabs
- No behavior change for when a new tab is added via the tab right-click menu for vertical tabs.
- Behavior change only for when a new tab is opened via the vertical tabs new tab button and the modifier + T keyboard shortcut, and then yes it would be inserted at the start of the list.

After a first pass at looking into this, I think it might be a matter of adding a condition in [_insertTabAtIndex](https://searchfox.org/mozilla-central/rev/261005fcc4d6f8b64189946958211259fb45e9e1/browser/components/tabbrowser/content/tabbrowser.js#3628).

So something like `index = this.tabContainer.verticalMode && !openerTab ? 0 : index` in the `else` block.
```
 // Ensure index is within bounds.
      if (pinned) {
        index = Math.max(index, 0);
        index = Math.min(index, this._numPinnedTabs);
      } else {
        index = Math.max(index, this._numPinnedTabs);
        index = Math.min(index, this.tabs.length);
        index = this.tabContainer.verticalMode && !openerTab ? 0 : index;
      }
```
But we'll need good test coverage to be sure.

Edited to include the modifier + T keyboard shortcut.