Opera's default search is actually Ask
Hit ctrl-Tab in Opera, and an Ask.com search input, is the first thing you see in the new Speed Dial window that opens. Most people use the search that is built into the chromework at the top right of the pane, of course, and Google is, indeed, in the dropdown list of options in that, search box. Uou can set it as your default search, but I can't recall whether it was Google, or infact, Ask, that was preselected at install time. Likewise, the search box in Firefox is actually a drop down, offering a list of options.
People tend to have these search boxes set to Google, simply because they want to be able to find the answers they need. Search isn't an ideological battle. Do a better job, and people will use your service. Before Google, everyone used to use Alta Vista - remember them?
Microsoft should really do more to get Live Search, or whatever they are going to call it next, added to the list of options in these browsers, rather than seeing the ownership of the browser as some sort of backdoor to owning the Web. The latter accusation tells more about Microsoft's intentions, than anything else. Sure, Google gets the top spot in Firefox, but then Google keeps forking money in Firefox's direction. It doesn't stop Firefox also having Yahoo and answers.com in it's list of alternative Search engines.
The real problem for Microsoft is it's track record, which iis what they are being judged on, here. On winning the browser wars, Microsoft did not respond with some huge Renaissance of ideas and fresh technologies: far from it. Instead, what we got was a near decade of stagnation and decline: a Web of Flash intros and kittens websites. Ten years on, and people are still getting excited about forms-based CMS, for goodness sake!
Microsoft doesn't want to own Web search because they have some serious and exciting ideas for it: it's so that Search can sit there in a corner of the screen in a dull grey rectangle, serving up apporoved content. The very best we could expect, is that there would be some registry hack, you could put in, to make the search look like a badly animated yellow dog.