SE doesn't support beta or alpha versions of web browsers, so bug reports that cannot be reproduced in release versions of any browser will be status-declined. There is usually some back-and-forth, with users asking the OP which browser he's using and then explaining that beta browsers are not supported.
I think it would be useful if anyone trying to submit a bug with a beta browser would be warned right at that moment about this, I'd imagine something like this:
This could reduce the number of spurious bug reports, and give the original user the opportunity to confirm the bug before actually submitting it.
Whether a user is using a beta version of any browser should be possible to detect via the user agent.