On developer edition here, no idea about Beta. Does the `about:config` option to disable signatures exist in your FF?
If so, make sure you grab the correct artifact to import[1]
[1] http://piro.sakura.ne.jp/xul/xpi/nightly/treestyletab-we.xpi
If so, make sure you grab the correct artifact to import[1]
[1] http://piro.sakura.ne.jp/xul/xpi/nightly/treestyletab-we.xpi
Edit: This was a really bad idea because upgrading to 57 destroyed my sessionstore.js. None of the backups were up-to-date for some reason. Be careful and make backups!