I like extensions. I've got 29 running right now. It allows you to have the functionality you need without the functionality you don't.
Also, Opera was designed to pass the acid2 test, while Firefox just added support wherever they thought it needed. Really, they both support standards about equally well, but I've noticed Opera doesn't handle mixes of old & new standards too well. I'd add a link, but the only ones I can think off atm would get me banned
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[OFFTOPIC]Question: If a link to questionable content is encrypted somehow (rot13 or 1337, like), would it be less a problem?
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Another reason:
#4: Themes. I have not seen the ability to use themes in Opera (or IE, for that matter). I like black japan, but it makes me paranoid.
[OFFTOPIC]I wish FF would let you change themes w/o restarting. I mean, trying out new themes is a pain in the :mad::eek::fu:, if you know what i mean.[/OFFTOPIC]
Overall, Opera is decent, but I prefer FF. Each to his own, though, as long as it isn't IE.
[OFFTOPIC]Sorry for drifting OT, my brain is asleep.[/OFFTOPIC]