Chrome has 15-20 times the users that firefox in the blackmarket the bug would sell for similar ratio. Safari might go for more as it has more rich and tech security illiterate users.

disagree. more marketshare does not mean juicier targets, which, in this case, would be tor users. in addition, you don't buy an exploit to use it en masse, that would get it burned really quickly

More market share does in fact impact availability of targets, but in the case of Firefox it's just as much a factor that there are more bugs and exploits floating around.