Well, I would hardly call it bloat...
IPv6 is a real standard. It will replace IPv4. It has to, simply because IPv4 doesn't have enough IP addresses in it to supply the needs of the future. But, the future ain't here for most of us.
Apparently many programmers decided that they want their software to be IPv6 ready so that (a) their software can be dropped in an IPv6 environment today without any issues and (b) their software won't become obsolete because it's tied to a doomed standard. The strategy that many have adopted is to have their software try IPv6 first, have it fail, then fall back on IPv4. Well this is fine and dandy for things like MPlayer, where you don't make a new connection all the time, but for a web browser it sucks.
As for Opera, now it has IPv6 support (before it didn't). However, Opera has IPv6 disabled by default (faster on the DNS resolves than a default firefox for IPv4, probably not for this switch thrown) so the people on IPv6 who want to use Opera are on the other side of the fence: some of them bitch because Opera doesn't work at all out of the box and they want it configured like Firefox is!
I guess what it boils down to is that IPv6 is a really really good thing to have, but I think the Mozilla people made a huge mistake by making it go IPv6 first, then fail down to IPv4 slowing down everybody but the small minority who have actually started to move to the new standard. Second, they made a mistake by hiding this setting in about:config and not documenting it well (lots of people have been frustrated by it for years and almost nobody seems to know what to do).
Something with this much of an impact on speed should be in the Preferences control panel.
By the way, if you do a search on google for this and find a post on a mozilla bbs with this solution in it by "guest999," that's me. I got to many forum accounts.
I hadn't really noticed the slowdown on my work computer before, because it has a really fast CPU. However, switching off the IPv6 DNS on firefox there sped things up there too... to the lightning fast level.