In about:config, the browser.tabs.insertRelatedAfterCurrent preference determines where links in a page open:

• true = next to the current tab
• false = at the end

Are you using Tab Mix Plus to manage tab settings in (much) more detail?

"browser.tabs.insertRelatedAfterCurrent" - is what I've mentioned that I changed... to true.

I'm using "Tab Control". I've just disabled it; and the problem remains. Other than that - nothing specific for tabs.

In Tab Control options, do you have this turned on:

"Insert unrelated tabs after current as well"

In about:config, that corresponds to this preference:

extensions.tabcontrol.insertUnrelatedAfterCurrent

It seems there was a recent change in Tab Control that separated that from the "related content" preference.

Yes it is marked; and in "about:config" - true. Something seems to be up with add-ons. Now several have a message next to them - "could not be verified". Though they work.

MrAccident said

Something seems to be up with add-ons. Now several have a message next to them - "could not be verified". Though they work.

In the future, Firefox will refuse to run unsigned extensions. However, the first step being taken in Firefox 40 is to warn you that you have some so you can check for updates and/or inform the publisher about this impending problem.

What the publishers need to do is submit their add-ons to Mozilla for review and signing some time before Firefox 41 is released and hopefully they will. If they don't, please complain to them!

Here is a different add-on to open "unrelated" tabs next to the current tab if Tab Control is no longer doing it: https://addons.mozilla.org/firefox/addon/open-tabs-next-to-current/ (good reviews, but I haven't tried it myself)

I actually already found this add-on after the problem began; but I just removed it today, because it screwed up some 200 and something tabs is a certain session!

But I didn't understand - shouldn't FireFox just open tabs next to the current tab? The problem started only about a week ago; so it has to be something they've done. And hopefully there is something to do about it.

By default, Firefox opens "related" content next to the tab, and other content at the end.

For example, a link in the current page that targets a new tab, or selecting text and using right-click > Search default-engine for selected-text should both open a tab immediately next to the page in which you take the action.

On the other hand, Ctrl+click on a bookmark will open a new tab at the end.

I don't know how Firefox decides whether an action of an add-on is "related" or not related.

Facepalm. I just unchecked "insert related tabs after current"; and now the search from the image search add-on - opens next to the tab. As with countless OS\programs related problems before - the answer is the simplest one. Thank you for your work.