Uninstall the Ask toolbar and it should work again. There is a compatibility issue with the Ask toolbar and Firefox that prevents new tabs from being opened.

There are a couple of places to check for the Ask toolbar:

• Check the Windows Control panel for the Ask Toolbar - http://about.ask.com/apn/toolbar/docs/default/faq/en/ff/index.html#na4
• Also check your list of extensions, you may be able to uninstall it from there - https://support.mozilla.com/kb/Uninstalling+add-ons

No program to remove from Add/Remove Programs. Followed the instructions on http://about.ask.com/apn/toolbar/docs/default/faq/en/ff/index.html#na4 to remove toolbar directly from toolbar. Removed all toolbars in the list and restarted computer. Restarted Firefox. Tab button now works. However, Ask.com reloaded with Firefox.

FYI - some toolbars may be installed by a 'partner' of 'Ask' (for example, mine was by 'Avery'). So, when looking in 'Add/Remove Programs' (or equivalent depending on your Windows operating system) don't just look for 'Ask' in the 'Program name' column, also look in the 'Publisher' column (I believe the list can be sorted on this column).