I can reproduce the issue.
(In reply to miralobontiu from comment #4)
When I dragged the carrot above the floating toolbar, the text was selected as expected. Maybe I did not understand the issue correctly.
My understanding is that, to trigger the issue:
- The page has to be scrolled down a bit, not scrolled all the way to the top.
- The caret needs to be dragged all the way to the top of the screen, so that the selection is extended to lines which are scrolled out of the view at the top, triggering "scroll into view" logic that should scroll the page (and the bug is that the page doesn't scroll).
My first thought was a regression from bug 1516056, but that doesn't seem to be the case: the bug reproduces at least as far back as the Firefox 66 release.
A regression window would be nice for further diagnosis (assuming this is a regression at all).