After about 15 tabs I believe, they become hidden on the right side and are accessible only by furiously clicking the "right arrow" and even then it takes forever and honestly this hidden tab feature seems to slow down everything. If you enter "about:config" in your browser without the quotes just set this value to get rid of this annoying (one of many new) feature from Firefox (set it to false and restart your browser): services.sync.prefs.sync.brows........