Hello,
I am using Firefox 4 on Windows 7, Flash is installed correctly and it works.
Sometimes, the graph of recent users is displayed, sometimes it is not, and then, Piwik complains about Flash 10 missing. All other graphs are displayed correctly, though.
It is also not a Firefox extension problem, since it works sometimes without any changes in the Firefox installation.
sometimes this happens with opera too …
workaround: remove the widget and add it again 
I also experienced the same problem
We’re going to replace the Open Flash Charts with JavaScript+canvas charts in 1.5. Watch for the release candidate announcements if you’d like to test it.
We need YOUR help! We are running a crowd funding campaign to raise funds to implement the detailed Visitors Maps of Countries, Regions and Cities (for all countries)!
These maps will be beautiful, usable, and built using open standards SVG+JS. They will show detailed visitor count, conversion rates, by Country but also (New!) by city and region.
Pledge now at: http://crowdfunding.piwik.org/analytics-maps-world-country-city-region/
There will be no more flash in Piwik and we will add amazing new Maps of your visitors.
Piwik needs you!