Although there is an add-on "tab killer" I wouldn't recommend it.

Many are usually at a loss when this question is asked; it's a bit hard to understand what the "problem" is. For example, you could use [shift]+[mouse left-click] to open a link in a new window instead of a new tab or do the same with mouse right-click and selecting "Open Link in New Window". Also, in Preferences > Tabs you could deselect the first option "Open new windows in a new tab instead", which should to some extent not open links in a tab that would normally open in a new window (I think e. g. a web page link that is designed to open in a new window - that's rare these days).

In other words, even though FF apparently doesn't allow you to turn off tabbed browsing completely, you are mostly free to use windowed browsing if you wish -- as a matter of getting used to using e. g. the mentioned features.

My company has a special problem with tabbed browsers. Our product is a SCORM compliant learning management system. Only one learning object at a time may be opened. Tabbed browsers make it possible to violate the one learning object restriction, so we have our customers disable tabbed browsing. The later versions of Firefox seem to have made it impossible to completely disable tabbed browsing.

Our customer base is very technically unsavvy, so we cannot tell them about doing control key sequences and the like.

I've tried to download Tab Killer, but it doesn't seem to want to download, and this forum seems to advise against it.

I know in most other forums I've researched, the people who try to help cannot understand why anyone would want to totally disable tabbed browsing, but we do have a very valid reason.

Our learning management system is provided by a vendor, so we cannot make any software changes ourselves, and to do so would violate the scorm compliance requirment.