Firefox 5 introduced auto-updating. I noticed that my installation of FF5 has already updated itself to version 6.
Does this mean that, like older versions of Chrome, Firefox 5 doesn't really need to be included in my cross-browser testing battery? Or is there a way some users may prevent it from updating that I should worry about?
You should look at the statistics when making such decisions. At this point, only 12% of Firefox users are using Firefox 5, with 55% being on Firefox 6. For comparison: Firefox 3.6.x is still being used by 20% of the users, Firefox 4 by 5% of the users.
I think that once the old branches (especially Firefox 3.6) expire you will indeed be able to concentrate on the latest Firefox release. The current tendency is that the previous release becomes irrelevant two weeks after its successor is released. While there will always be some who turn off auto-updates, the numbers are pleasantly low.
Source of statistical data: https://addons.mozilla.org/en-US/statistics/addon/1865. Feel free to compare with other sources.