See my response (below), to a later thread.
Did you enable the "Do Not Display Again"option for the "Do you want to run the plugin?" prompt and then select "No" for the prompt? In which case, you permanently disabled all Adobe Flash plugins (for current XMB account), hence no video playback.
Luckily, I didn't fall into this trap, added by Firmware 4.30 (Flash adverts don't bother me) and all Flash video that previously worked is still working fine, such as YouTube.
If you did indeed fall into this trap, I would recommend using another XMB account to view Flash videos as I have seen several reports that using "Restore Default Settings" causes more serious issues, such as breaking games downloaded from PSN (is this still the case?).
P.S. 4.31 doesn't help with this issue.
All my games are fine even after a restore default settings.
Hopefully a few more users, who used Restore Default Settings to fix this issue, can indicate whether they experienced any nasty side effects, especially games download from PSN no longer working. The side effects may well have been fixed some time ago, but if I'd downloaded a ton of content from PSN I'm not sure I'd be willing to take the risk (given the previous reports).
Tribble6..... Thank you so much bro. Follow your advice, I fixed the problem just like that. I logged in as a new user and did not fall in the trap as I clicked NO when it says O YOU WANT TO PLAY PLUG IN". Again thanks TRIBBLE5 and Everyoone!!!!!!
As I reported in my browser thread, there's actually a full solution (may well require installation of optional 4.31 Firmware update):
To restore the "Do you want to run the plugin?" prompt, all you need to do is to delete your browser's history (forget deleting cookies etc)! If you are trying this on page with plugin, you can Refresh page once done.
If you are new to the browser, to delete History, press Triangle and select History from menu and then press Triangle again (to display Options) and select Delete All from menu and Yes to confirm.
What had been throwing me here is people were saying you needed to delete everything (cookies, cache etc) and you delete everything except History in Tools menu, so I had just been deleting everything under Tools menu (including Search History) and nothing happened. As indicated, all I actually needed to do was to delete history...