When selecting text in a text field with only one column, and you move your mouse above it, the selection will jump to the beginning. This is how it should be: https://gfycat.com/QualifiedPalatableCanary This is what it looks like in Firefox or Chromium for example: https://gfycat.com/PeacefulFondAlaskankleekai
While I do not agree that the jumping behaviour is correct, I suggest to report this to Qt developers, because the mouse handling in QLineEdit is inside the Qt libraries. https://bugreports.qt.io/ Btw, that's "one row", not "one column".
Ah wait, you are saying that Dolphin is correct, and Firefox is not? Then you should report it to Firefox developers.
To narrow it down.. it happens in: Firfox Chrome Steam Thunderbird qbittorrent Vivaldi ArduinoIDE Atom Works as expected in: mumble dolphin octopi libreOffice