This has been such a pet hate for years on systems with a mouse as well as iPhone iOS, that I assume it is nigh impossible to select text reliably and conveniently, for some unexplored reason or other. It's always the case that I can get a selection backwards up to the first letter but missing it, or including the first letter and some extra junk from the lines before, but not simply up to and including the first letter.

Right now replying to your comment on Windows, FireFox, with a mouse, I have just discovered a new text selection mode I've never noticed before[1] - double-click-and-drag selects whole words. Consistently up to the new word boundary on the right. That means it's possible to select an entire sentence up to (but without) the period. Or including the period and a trailing space. But not the obvious one to want - a sentence including the period and no trailing space.

[1] I know double-click commonly selects words, and triple-click selects sentences, but didn't know of double-click-drag.