Horizontal scroll bar appears when content is larger than the box itself.
Since the latest Firefox update, when working in a form, if the text entered into a text box is longer than the size of the box, then a scroll bar appears which hides the text behind it. In the old version, you could simply select the text and drag right to see the rest of the text.
Re: Horizontal scroll bar appears when content is larger than the box itself.
I haven't noticed this in <input type="text">. Does it only happen on particular sites?
You can check what kind of form control it is by right-clicking > Inspect element. Use the button with the 3 offset horizontal bars to see the HTML tag in context. You can right-click to copy it. (If you click the Style button at the lower right, you also can view the style rules for the field.)