Post a bug in JIRA, include a sample project if you can so we can look at the problem easily.
Only a year late, but I've added some comments to this JIRA. I can reproduce the issue.
Basically as far as I can tell it's a Firefox only bug and seems to only occur if you reRender the form that contains the button/link with the s:defaultAction.
Workaround is to reRender elements within the form instead.