[rjprins]

I would simply use autohotkey:

    ^{Right}::Send ^{Tab}
    ^{Left}::Send ^+{Tab}

But then, that changes behavior for all applications, not just Firefox.

There is a keyconfig extension for Firefox: http://kb.mozillazine.org/Keyconfig_extension you could try that.

In Opera it is easy. Personally, I have this functionality under 'j' and 'k'.