I use it to remap "Previous / Next tab" (via "option-command-n" and "option-command-e") and it only works after a website is "loaded" and it doesn't work on all tabs (for example: it doesn't work on the "Add-ons Manager" tab or other system tabs).
Unfortunately, as of now this is a limitation of Firefox API. A new version is coming that moves the loading of the addon a bit sooner, but the page needs to be mostly loaded for it to work.
