If you select a link within the current tab Window "open new tab" that will be next to the current tab your in but if open a new Windows not related to any tabs it from my usage opens it at the end of the tabs. So it could be 20 tabs longs and it will become the 21 tab in the list.

Note that this is about that the Tab bar scrolls the current tab to the far left when a redraw is initiated by some operations like opening a new link and there are that much tabs open that the Tab bar scroll buttons appear. I'm not aware of a way to prevent this from happening although this shouldn't happen if you open a link next to the current tab.

Are you possibly using extensions that could affect the way to open tabs?

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems