I agree, there's definitely flexibility in what I expect, and what other people will likely expect. Here's the thing though: when I double-click "double-click", I could reasonably expect "double" "click" or "double-click" would be selected. I wouldn't expect it to select everything between the last and next punctuation characters (as an imperfect analogy).
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.