Because Opera is not a serious browser to worry about maybe?
Fewer and fewer people are happy to pay for an internet browser, when
mozilla was a big ugly ram eating dog then maybe but now firefox is a
serious contender and more and more people are switching to that.
Operas real power lies with mobile debives not the desktop, and google
has probably looked at the stats and decided to support the major
browsers first. Firefox in itself is not a major browser, but the fact
that Netscape/Mozilla/Firefox all share the same backend gecko
rendering engine means support one mozilla derivitave, support them
all.
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Regards,
Sascha