There definitely should be a + button immediately following the last open tab on the tab bar. If you have so many tabs that they "overflow" the available space, the + button should appear after a ">" button.

To try to restore your tab bar to its normal layout, you can try the Restore Defaults button in the Customize tab. This will rearrange your toolbar buttons to factory positions, but add-ons which modify the bars (either by adding their own icons or by modifying Firefox controls) will still take effect after the change. You might not see it in that window, but if you launch a new window (Ctrl+n) you should see the combined effect.

This article describes the Customize tab: Customize Firefox controls, buttons and toolbars.

Does "Restore Defaults" bring back the + button?

As an aside, one other way to open a new tab as a temporary workaround is to press Ctrl+t on the keyboard.

By the way, I notice you have the iCloud Bookmarks extension (listed under Question Details > More System Details next to your question). Other users have reported crashes with this extension so you might want to disable it to see whether that makes any difference. You can do that on the Add-ons page, either:

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

In the left column, click Extensions. Then on the right side, if you can live without it for a while, disable it.

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.

Is the Firefox window maximized?

Try to set browser.tabs.drawInTitlebar to false on the about:config page to disable displaying the tabs in the title bar.

dit00@hughes.net said

The only way I can get a new window to do a new searh is to go to edit click on new tab.

I tried all the things suggested but it did nothing for the problem. I fixed my problem by bringing up a different browser and searched for the problem that was not known. The only reasonable suggestion was to reset Firfox and start all over.

     This is about the 10 time I have had to resort to the reset/restart way of cleaning up Firefox and getting it to run again. Not a very good track record to date. My opinion of Firefox is that when it works, it's the best there is, but it's instability is still open to question. Right at the moment Firefox is working, but for how long.
    As a result I am experimenting with MS Edge to see if it is anymore stable. Stability is my primary requirement. All these little extra gimmicks are cute but I don't care.
         Dave Tomlinson

Hi Dave, I don't understand why your Firefox keeps breaking. One possibility is that there is corruption in the program files, which is easy to address using the steps further down in this reply. Another potential scenario is a bad add-on reinserting itself post-Refresh.

If the problem recurs, please try this:

Test in Firefox's Safe Mode. In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(A) Download a fresh installer for Firefox 43.0.1 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(B) Exit out of Firefox (if applicable).

(C) Rename the program folder as follows:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Does a cleanly reinstalled Firefox start up and run normally??

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins