Firefox, in contrast, breaks at script boundaries, so it'll select runs of Hiragana, Katakana, and Kanji. Not nearly as useful, and definitely makes copying Japanese text especially annoying.
It's not like English highlighting does complex grammatical analysis to make sure "Project Manager" gets highlighted as one chunk and "eventUpdate" gets highlighted as two chunks, most implementations just breaks at spaces like the user expects.
Consistent behavior > inconsistent behavior, almost always. If I can anticipate what my computer is going to do, I can plan around it, even if it means a bit of extra manual work.
In general, whitespace characters have no place or significance inside a Japanese sentence, and most of the whitespace in Japanese typesetting is built into punctuation marks.
Furthermore, even most punctuation is optional in Japanese. The full stop 。 and comma 、 are mostly a matter of preference, sometimes spaces are used in place of full stops or commas.
Firefox's approach is fairly useless in this regard. Even if it's predictable from a technical perspective, it's not predictable for a reader who naturally processes semantic breaks rather than technical ones. Unlike in English, where a space is both semantic and visual, hiragana-kanji boundaries often don't mean anything. As a result, for me at least, Firefox's breaks feel a lot more random than Chrome's, which, while dodgy, are often fine.
Having used Firefox as my main browser since 2006, I remember when I discovered this feature in Chrome, and being shocked at how much of an effect that minor improvement had for me. It's not a deal-breaker, certainly, but it's become my one big annoyance with Firefox.
Machine-readable strings can still just be an ASCII byte array, but we need to keep the two separate.
[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.]
Also FWIW Safari also supports this.
I don't know Alt + Shift/Delete action, how is it useful?
Now consider another anglophone arbitrarily; how likely does it seem to you that they would share the exact same set of expectations of the above with you? Because if they don't, then one of you is going to have their expectations violated.
Now give it a try: do the highlights match your expectations? (Moreover: are those expectations consistent with where you placed the word boundaries?)
If your expectations of what double-click highlight does for English is consistent with what actually happens when you double-click, it is deeply unlikely that the causality goes word boundary -> highlight expectation; if they do, it's more likely that double-click highlight has influenced your sense of word boundaries.
But it's even more likely that you your expectations of double-click highlight don't actually match up with your intuition for word boundaries, and you just have a pretty good model of how double-click highlight behaves independently of where you think the word boundaries are.
(The notion of word boundaries in this context is fraught anyway. Even setting aside the problem of compound words, the phonological word boundaries of Japanese don't match up with the lexicographical ones, in both directions.)
For me at least, double-clicking is like fuzzy string matching. It's an imprecise behaviour by nature, but should be designed to be useful. Firefox's behaviour in the case of Japanese, but especially Chinese, often isn't useful, not because it doesn't perfectly reflect linguistic expectations, but because it usually selects too much--meaning you can't correct your selection by moving the cursor back and forth, and instead, have to stop and select using a different method.
Take the following passage:
もうこんなことは終わってほしいと願うばかりだ
Different people may break apart もうこんなことは differently, but it's hard to argue that it's one word. Chrome (and Word) separate it like this: もう こんな こと は, and whether or not you agree that each one of those parts are words, simply splitting もうこんなことは into parts semi-logically makes it smoother to edit (for me, at least! I understand most people might not be as dependent on double-click selection).
The same behaviour for Chinese is so useless that it's not even worth mentioning, since you'll always select either a large part of a sentence, or an entire sentence at once. I mean, I love the word 匹配的近似度用如下方法来度量.
Firfox is just not a very good macOS citizen, sadly.
If I could set up a custom keyboard shortcut for the service, it would work great—but support for custom keyboard shortcuts also doesn't work in macOS Firefox[1]! (Unless you set a combination which Firefox doesn't already use for something, and it already uses everything sensible, so you have to set something long and annoying.)