What I do: Type text, anywhere in Firefox: Address bar, search bar, forms in websites. What happens: Text appears. It is however "non-existent" in that it does not trigger any UI reaction and is never submitted or accessible via JavaScript. The cursor is also missing. The issue persists until Firefox is restarted. What should happen: Text appears and "interacts" with the UI and is submittable as well as JS-accessible. The cursor is visible and blinking. Notes: This appears to be related to Adobe Flash. It only happens after interacting with Flash applets. It is, however, not really reproducible but random. I have also deactivated OOP execution for Flash (and only for Flash) because of performance issues.

Sounds like bug 718939. We are building a 10.0.1 that may fix the issue.

Right, seems to be it. Hopefully, this will be fixed soon. :D