This problem in is Firefox on your Mac?

May users have discovered this problem is caused by Conduit add-ons, such as a Conduit plugin and/or a "Community Toolbar" extension.

You can check for and disable extensions on the Add-ons page. Either:

• Command+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Does that help with the "+" button? If so, could you name the specific extension that caused the problem?

You might also check the Plugins section of the Add-ons page. You can use "Never Activate" to disable plugins that you don't ordinarily use.

Thank you, jscher! Yes, I use Mac, and yes - the problem was exactly what you said. The offending extension was that Community Toolbar. When I disabled it, I could make Tabs again. Interestingly, my other computer did not have this extension, so of course it worked fine.