Option to prevent tab strip from auto-scrolling to new tabs opened from pinned tabs

[linkp56]

When opening a link from a pinned tab (e.g., middle-click or right-click > Open in new tab), Firefox opens the new tab at the far right of the tab strip and automatically scrolls the tab bar to reveal the new tab.

This behavior is distracting and frustrating for users who:
- Keep many tabs open, which makes the tab strip scrollable.
- Use pinned tabs as web app launchers (e.g., Gmail, calendar, task boards).
- Prefer to stay focused on the left side of the tab bar.

🔧Feature request: Please add an option (or change behavior) so that the tab bar does not scroll away from the user’s current location when a new tab is opened from a pinned tab — even if the new tab itself appears at the end.

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當從固定分頁（例如中鍵點擊或右鍵→在新分頁開啟）打開新分頁時，Firefox 會將新分頁開在最右側，並自動將分頁列捲動過去顯示新分頁。

這會干擾使用者的操作視角，尤其是：
- 有很多分頁，使分頁列必須水平捲動。
- 習慣將固定分頁當作應用啟動器（例如 Gmail、日曆、任務板）。
- 長時間操作時視線集中在左側的固定分頁區域。

🔧建議功能：希望新增一個選項或調整行為，讓從固定分頁開啟新分頁時，分頁列不要強制捲動到新分頁位置。即使新分頁仍然開在最右邊，也不應打斷目前的視角。

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Suggested technical note:
- `browser.tabs.loadInBackground = true` helps avoid focus, but doesn't prevent tab bar scrolling.
- May relate to tab animation or visibility logic in the scrollable tab UI.
- Possible pref name: `browser.tabs.preventScrollOnPinnedOpen`.

[Jon]

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