If you mean “link” when you say “tab”, maybe this will help.
http://www.ghacks.net/2009/07/03/force-firefox-to-open-links-in-same-tab/
Thanks, but I did mean tab. It only happens on rare occasions when I attempt to open a tab. 9 times out of 10 the tab will open as it should (in the same multi-tab window, not in a brand new window). I checked the long list of reported bugs on the Mozilla site I linked to above and couldn’t find anything that sounded like it, although most of the bug descriptions were incomprehensible.
For the ‘bug’ link, see:
“....filing a bug in Bugzilla.”
It’s a hyperlink at the end of the first paragraph.
http://www.mozilla.com/en-US/firefox/3.5.1/releasenotes/