You can download this extention http://forums.mozillazine.org/viewtopic.php?t=72994
note: the page looks lousy and dated, but the tool works.
once installed, go to menu 〖Tools ▸ Keyconfig〗.
by default, there's no key setup for prev/next tab. You can add it, by clicking on the “Add a New Key” button. Here's the code:
Once you added, click on the key box and press the key you want.
Also, be sure to click Apply.
Then, it'll work. No need to restart after changing keys.