see

https://support.mozilla.com/questions/863610

See item #4 in the following set of instructions

You can make Firefox 7.0.1 look like Firefox 3.6.*, see numbered items 1-10 in the following topic Fix Firefox 4.0 toolbar user interface, problems (Make Firefox 4.0 thru 8.0, look like 3.6). Whether or not you make changes, you should be aware of what has changed and what you have to do to use changed or missing features.

• http://dmcritchie.mvps.org/firefox/firefox-problems.htm#fx4interface

There is a lot more beyond those first 10 steps listed, if you want to make Firefox more functional.

Please mark "Solved" one answer that will best help others with a similar problem -- hope this was it.

You can find the New Tab button showing as a '+' on the Tab bar.
You can open the Customize window and drag the New Tab button that shows as a plus sign (+) from the Tab bar on another toolbar and it will become a regular toolbar button like the New Tab button that you have in Firefox 3 versions.
If you can't find the New Tab button then click the "Restore Default Set" button in the Customize window.
If you want the New Tab button at the far right end of the Tab bar then place a flexible space to the left of it.

• http://kb.mozillazine.org/Toolbar_customization
• https://support.mozilla.com/kb/Back+and+forward+or+other+toolbar+items+are+missing