Today, Firefox 5 was released. If all goes according to plan, Firefox 7 will be out by the end of the year. Firefox has adopted the Google Chrome development model wherein version numbers are largely unimportant and so just supporting "the latest (publicly available) one" is probably the best strategy.
But how do you best test that? As my QA guys have pointed out, if you tell the client that you support "the latest version" but a version comes out that breaks your site, then you have a problem because now you've stated you support a web browser you don't. And since both Firefox and Chrome now update themselves automatically, the average person probably has no clue or care what version they're running. And having them either not upgrade or roll back is nontrivial.
I'm finding there are a number of organizations that mandate their employees use IE (the head of IT subscribes to the Microsoft school of thought), or mandate their employees use Firefox (the head of IT subscribes to the IE-is-insecure school of thought), so Chrome updating constantly was a non-issue. But now that Firefox is a member of that club, I can see this becoming a bigger issue soon.
My guess, in the case of Firefox, would be that the Aurora channel is the key, but what is the best way to approach testing it? Should we fix anything that comes up as an issue in Aurora, or should we wait until closer to the scheduled release? Do people automate this sort of thing?