I've seen two things implicated in several google support threads about the issue. First, having both the system and google's sandboxed flash versions enabled as plugins, and second, an older version of Trusteer Rapport (typically offered by banks) was implicated.
Both may be fixed by now, but if it's still crashing, try about:plugins, enable Details at top right, and disable the system version of Flash.
What seems rather ironical is that I only started having these issues with Chrome after they added the Flash-sandbox, which was supposedly made to stop Flash from crashing the browser. With the result that Flash is always crashing the browser, something it never did before.
Sometimes I think Google needs a bigger QA-team.
When I visited Google Zürich (admittedly not as crucial as Mountainview) I learned that unit testing was mostly used so they had pretty red/green lines on those LCD screens in the hall, and that any dev could break the version /in production/ pretty easily with a misplaced commit...
This is not about preventing Flash from crashing, but about preventing your machine from getting owned by an exploit for one of the countless discovered and unpatched flash security flaws.
> ... I've moved back to Firefox 4 ...
That would be great, except FF4 doesn't let me log in to HN.