This page shows you how to add or change Firefox keyboard shortcuts.
Download this Firefox extension at http://forums.mozillazine.org/viewtopic.php?t=72994 (That page looks lousy and dated, but the tool works.)
Once installed, go to menu 〖Tools ▸ Keyconfig〗.
by default, there's no key setup for previous/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.