I'm Japanese and I agree that Firefox behavior makes sense.
For example, a text with kanji like "その機能拡張は、", A word "機能拡張" consists of "機能" + "拡張" words.
In Chrome, double-click makes individual part (like 機能) selected which is rarely wanted behavior for me.
In Firefox, the whole word (機能拡張) is selected, which is wanted mostly.
It's pretty painless to select the next word though. It's best for the computer to err on the side of caution in this case.
[Edit: Ah, it might not be common knowledge but you can drag on the double click to select more words. You can also hold alt while editing text to jump words, and even combine alt+shift and alt+delete. Windows and Apple differ on the boundaries wrt whitespace, but I'm fairly sure they're both alt. Wordjumps while editing is an unappreciated superpower imo.]
I know drag after double click action but I don't use double click to select action for Japanese text and always select text by dragging because I prefer predictable operation.
I don't know Alt + Shift/Delete action, how is it useful?
