Single Key Tab Switch is not compatible with the "Search for text as I type" feature in Firefox. (see issue #1) When this feature is enabled, the add-on will simply not work, as explained in the bug report.
Source code can be found here: http://github.com/oyvindio/skts
Bug reports can be posted here: http://github.com/oyvindio/skts/issues
Note that posting bug reports and/or questions in reviews is not really a good idea, as it is kind of hard to reply to a review. Feel free to open an issue on GitHub if you want to report a bug, or drop me an e-mail with your questions, and I'll try to get back to you.