When I click on a link, it opens in a new tab, FF3.6, would open a link in the same tab, unless Iwanted it in a new tab.

Question

Any link that isn't a button, opens in a new tab.

Answer 1

I don't know of this behavior being changed just by going to Firefox 4.0, basically this is difficult to do if you want that behavior.
'''Middle-click''' defaults in Firefox to opening a link in a new tab (I override through the system to be "Enter")
Do the links open '''into separate tabs''' or do they open into the '''same different tab''' (like in Google searches).
Does this happen from any web page. Normal preferences (but maybe not default) used from search bar, and search results.
Firefox 4.0 introduced '''app-tabs''' which are pinned or unpinned from the tab context menu (right-click on tab). A pinned tab would not show the title on the tab and shows the favicon centered on a narrowed tab. Links opened from pinned tabs open in a new tab, does not apply to use of keyword shortcuts -- not links.
"'''Perma Tabs Plus'''" extension could do that but you would have to invoke it for each tab, though isn't of much/any use in Firefox 4.0 although it did work differently it is or was not working same with Firefox 4.0 (though I still have it active).
"'''Tabs Mix Plus'''" fiddles around with a lot of things almost anything could happen and it could leave things permanently switched (if you have it check it's options; I would never recommend the extension).
Try bringing up Firefox once in "'''Safe Mode'''" to see if that behavior disappears. Firefox 4 can help you with that quickly though Help ("Alt+H") -> "'''Restart with add-ons disabled..'''." this method will bring up safe mode for only the one time, then back to normal. This may not affect all things changed in "Tabs Mix Plus" but otherwise if things work properly in safe mode it generally means an extension was involved.
If an extension is involved the first place I look is "'''[http://kb.mozillazine.org/Problematic_extensions Problematic extensions]''' " then the process of whittling down possibilities by disabling/reenabling half of enabled/disabled extensions described in ''' Extension issues''' within "'''[http://kb.mozillazine.org/Standard_diagnostic_(Firefox)#Extension_issues Standard diagnostic - Firefox - MozillaZine Knowledge Base]''' ".