For my own reference, really. And because the keyconfig.xpi download link seems to jump around page numbers on Google’s search results.
The original download is at http://mozilla.dorando.at/ (and my mirror).
The binding to switch to the next Firefox tab is:
gBrowser.mTabContainer.advanceSelectedTab(1);
And previous tab:
gBrowser.mTabContainer.advanceSelectedTab(-1);
If you want them to wrap around in the tab order, then use gBrowser.mTabContainer.advanceSelectedTab(1,true); and gBrowser.mTabContainer.advanceSelectedTab(-1,true);.