You can move the tabs bar back to the bottom by clicking the orange Firefox tab at the upper left hand side of your screen. Click options then remove check in the "tabs on top" box. That should put you tabs back on the bottom. If you don't like it then you can simply change it back. Hope this is what you were trying to do.

It's on the toolbars pop-up. Right-click (Ctrl+click) a blank area of the Tab Bar or a blank area after the address box on the Navigation Toolbar. Uncheck "Tabs on Top". If you don't see that menu, try View menu > Toolbars.

@rickm1945, that should work on Windows or Linux, but for some reason, Mac users have a menu bar instead of the orange Firefox button.

Thanks guys! got it!  :)

After reading the posts here, it wasn't hard to move the tabs to where they were before I upgraded to Firefox version 5.0, so thanks to the bods here for telling me what (in retrospect) is blindingly obvious, but why on earth do settings like this change without the software bods informing the users: (a) what will be changed and (b) how to reverse these changes if they're not wanted? Even better would be if users were given options before upgrading so they can choose for themselves whether to allow these changes or not. Well that's my opinion anyway, for what it's worth.

I have been avoiding the upgrade because I didn't like the new layout with the tabs at the top... I may try it now. Thanks.