I have not heard of a feature to move an existing tab from a particular site next to a new tab from a particular site. An add-on could do it, but none of your enabled extensions sound like they would have that feature.

Was "Tab 1" a pinned tab (also known as an app tab)? Pinned tabs are always at the left end of the Tab bar and show only the icon, none of the page title, and don't show an X close button. If a pinned tab suddenly became unpinned when you opened a new tab to the same site (was it the identical URL?), we could think about why that might have happened. I don't use pinned tabs myself, so I don't have an instinctive sense of their behavior.

The tabs move to the right from the tab you open from. This is how open tabs work. This is how it's been since I used firefox. I can drag and drop tabs once opened to where I need it.

Regarding not being able to move tabs: have you exited Firefox and restarted it since this problem began? As far as I know, there's no feature to lock tabs in place, but there could be a transient problem with a particular window that continues for the duration of your session.

If it's critical not to lose your current session tabs, BEFORE EXITING, please make a backup of your session history files as follows:

(1) To open your profile folder...

You can open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Open Folder" (or "Show in Finder") button.

(2) Copy out session history files

In your profile folder, double-click into the sessionstore-backups folder. Save all files here to a safe location such as your Documents folder.

(3) What are all these files?

The kinds of files you may find among your sessionstore files are:

• recovery.jsonlz4: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.baklz4: a backup copy of recovery.jsonlz4
• previous.jsonlz4: the windows and tabs in your last Firefox session
• upgrade.jsonlz4-build_id: the windows and tabs in the Firefox session that was live at the time of your last update
• various .js files from Firefox 55 or earlier

Most likely only recovery.jsonlz4 will be interesting.

I can now move the tabs again. Maybe it was just a transient problem. I had several tabs open and still do.

WestEnd must have misunderstood me. I said, "When I opened a different gmail account (Tab2) in a new tab both Tab 1 and Tab 2 moved to the right side of the line of tabs." I meant that Tab 1 and Tab 2 both rent from the Left side of 6 or 7 other tabs to the right side of all of them.

They were in this order before I opened T2:

T1 T3 T4 T5 T6 T7 T8 T9

After I opened T2 they were suddenly in this order:

T3 T4 T5 T6 T7 T8 T9 T1 T2

I am sorry you misunderstood WestEnd.