To get Flash 10.3 working on the stock browser you need to do two things:
1) force stop the browser process
2) Remove all permissions for /data/data/com.android.browser/app_plugins/com.adobe.flashplayer/.macromedia/Flash_Player/config.data
*Fix also works for Dolphin. Just change com.android.browser to mobi.mgeek.TunnyBrowser
Changing the permissions restricts the flash plugin or the stock browser or the webkit or I dunno what from reading config.data. This file is screwing with video playback of anything over 240p.
What's weird is that the config.data generated for 10.2 and 10.3 appears to be identical, except for the ProductVer string. Since the file contains settings for h264 high, baseline and main profiles, there's probably some change in 10.3 to the way it uses this information.
Either way, this is a bandaid for those that don't wish to use Opera until the heart of the issue is found/addressed.