Since no one else felt the need to answer, I found a work-around. The problem went away when I unchecked the "Enable multi-process Firefox Developer Edition" box on the "General" section of the FF Dev preferences. Apparently, Mozilla's latest brainstorm still has bugs.

Keep i mind that the Aurora aka developers edition (a2) builds you are Testing gets checkins almost everyday so they get updates each of those days. This can can sometimes lead to issues until fixed, finished or reverted. The same is the case with the more unstable Nightly (a1) channel builds.

Most posters here use the Release and Beta builds at most so a place you could hang out with Testers if you want to test Aurora and or Night channel builds is at http://forums.mozillazine.org/viewforum.php?f=23

Note new members there requires first post approval and then can post normally. This is due to spam attempts mozillaZIne gets each day.

Thank you for the effort, but the reason and the only reason I posted here was this was where the "Firefox Help" entry took me. I would not be using the Developer's Edition if Mozilla would allow me to use "Tabkiller"; it is locked out in the full release because it is "unverified" and they insist on opening EVERYTHING in TABS! I'm willing to put up with the instability of Developer's Edition just to avoid the tabs.

Piro the author has stated on http://piro.sakura.ne.jp/xul/_tabkiller.html.en that the version on https://addons.mozilla.org/firefox/addon/tab-killer/ is signed. You may need to reinstall extension perhaps.

There has however been no new versions of this Extension from the author since the last update way back on April 26, 2007 as you can see at https://addons.mozilla.org/firefox/addon/tab-killer/versions/ and based on version number as it is the date.

Thanks again, but it refuses to load in a newer version of ff or ff dev.