You definitely should not need an extension to get a new tab button! Do you have any tab related extensions now, such as Tab Mix Plus or Tab Utilities?

Try resetting your toolbar layout by renaming the file that stores customizations. Firefox should returns to the default settings for toolbars and windows. Here's how:

(1) Open your personal settings (AKA Firefox profile) folder

Help (or Alt+h) > Troubleshooting Information > "Show Folder" button

(2) Switch back to Firefox and exit the program (e.g., File > Exit)

(3) Wait a few moments for Firefox to finish updating files in your settings folder, then rename localstore.rdf to something else, like localstore-bad.rdf or localstore.old. Alternately, you can delete the file.

(4) You're done with your profile folder and you can restart Firefox now.

Any luck?

With respect to links launching in new windows instead of tabs, the checkbox in the Options dialog should work but there is an exception for scripted windows that have certain parameters set (e.g., dialog style with no toolbars, or a specific size). Launching a link out of a dialog, which is not allowed to have extra tabs in the window, also will launch either a new window or be added to an existing window. Any chance the problem is related to those situations?

Thanks for the quick reply. The issue mysteriously fixed itself. I closed out multiple times and it finally started working as normal; although, some of the formatting of the menu items is off. The only thing I can think is that a recent add on I downloaded screwed it up momentarily. I added SEO toolbar just before this happened.

And once again the issue returns. I have not changed a thing but the + tab button is gone again. I have the options set correctly as well. This is a pain.

Try disabling all recently installed, nonessential, or unrecognized extensions:

orange Firefox button (or Tools menu) > Add-ons > Extensions category

After restarting Firefox, do the toolbars return to normal? You can selectively re-enable extensions as needed.

That did the trick. I'll enable each individually until I find the problem and post the results.

Thank you.