Is it changing the current tab, or the current window?

Good point. Just the tab, so that if I wish to revisit the site/tab left or right of the current window, I have to scroll either direction. Super annoying! Has been occurring since I received my Acer Aspire SW5-012 over 1 year ago, which came loaded with Win 8.1. I also imported my previous bookmarks library, which originated many versions ago using Ubuntu and worked fine on WinXP.

Thanks for your reply. Hope you can solve.

I shall try.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

If you open a link from a page and have set to open new tabs after the last tab (browser.tabs.insertRelatedAfterCurrent = false) then Firefox either scrolls the current tab to the far left or sets focus to the newly opened tab. In case of the former you should still have the current tab visible.

Thank you FredMcD. My problem continues despite starting Firefox in Safe Mode (I since only changed Firefox's default by allowing viewing of the menu bar). In observing closer the active tab does shift far off out of view to the left at seemingly random moments (switching between applications or visiting other pages of the same website).

I discovered long ago the half work-around of bringing back the active tab into view by opening a new tab/page from the current active page, whereby the setting would automatically switch/"set focus" onto the new page as explained by cor-el. Weeks ago I reset the opening of a new link from a current page to not automatically switch, presumably also Firefox's default. However, the original problem persists. I especially dislike when I want to close a slow or bothersome site and the tab has danced off out of view; and the option of opening a new tab defeats the purpose I have in mind or worse, causes Firefox to freeze and leading me to shut down the browser.

Thanks for your feedback.