Users have reported that this new tab dysfunction is associated with 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.

Does that help at all?

If you have a website add a bookmark to Firefox, due to restrictions on that feature, they set it to open in the sidebar.

To fix that, right-click the bookmark and choose Properties. At the bottom of the Properties dialog, uncheck the box for "Load this bookmark in the sidebar".

Does that fix it?