These are regular extensions from the Mozilla Add-ons site?

Is the extension opening a pinned tab? Those always cluster at the left. Maybe there is a setting to not use pinned tabs.

If it's not a pinned tab, could be it a "related" tab? Normally links opened from a tab will open immediately to the right of the tab to which they are related. Could that be the reason the new tab might open to the left of the currently active tab, because it is "related" to some other tab?

jscher2000, thank you for your answer. Yes, these tabs are 'pinned' tabs and opened by standard plugin 'Distill web monitor'. Any way to change position of pinned tabs ?

Plugin has option to change the method of querying network, but other methods are even worse...

I'm not aware of a way to move pinned tabs from their regular position to another position.

Recently they updated Distill Web Monitor to be incompatible with the Firefox 62 release, so probably you are running the original release from May. Presumably there will be an important feature change that will take effect in Firefox 63 (currently in beta testing), but I have no idea whether it will help with this issue.

https://addons.mozilla.org/firefox/addon/distill-web-monitor-ff/versions/

I suggest asking them about how to address this. It would be fine if there was a single persistent tab or a regular non-pinned tab, or if they used a background window, but it's sort of senseless to keep opening and closing pinned tabs in your currently active window, if that's what they are doing.

jscher2000, You are right, persistent pinned tabs (if possible) probably will fix this visual distraction, I'll contact plugin developers. Thanks!