I feel like this is a conclusion you could only reach by having an irrational compulsion to defend the deficiencies of Firefox, and not being a regular user of the Japanese language.
I don't use/speak Japanese, but the GP's conclusion makes complete sense to me.
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.
That doesn't work if the consistent behaviour is completely useless. If double click selected English tokens grouped by which half of the alphabet they came from, nobody would ever double click anything, they'd just click-drag highlight.
Spaces have zero importance to the Japanese language itself, but they are occasionally used like punctuation. e.g. some YouTube video titles will use spaces around names of things that could be hard to parse as not part of a sentence, another example is when you're typesetting phrases in lyrics.
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.
