Hmm, what could cause that? Ctrl+A (Select All) or Shift+Home (select to start) would select the text in the field, but both of those would be quite difficult to type accidentally. Is Firefox showing any suggestions in a drop-down on the field that might be a factor? Does it seem that Firefox might be losing the focus at any point, like quickly switching away from Firefox to a different app and then back again? I guess if you aren't losing any keystrokes, probably not. As a test, you might just type asdf over and over and keep your eyes on the tab bar. The text will be faded if Firefox loses the focus (if that ceases to be the active window).