Install/Update Adobe Flash Player for Firefox (aka Shockwave Flash): your ver. 10.0 r42; current ver. 10.1 r53 (important security update 2010-06-10)
~~red:Check your version here~~: http://www.mozilla.com/en-US/plugincheck/
See: Updating Flash
-use Firefox to download and SAVE to your hard drive (save to Desktop for easy access)
-exit Firefox (File > Exit)
-check to see that Firefox is completely closed (Ctrl+Alt+Del, choose Task Manager, click Processes tab, if "firefox.exe" is on the list, right-click "firefox.exe" and choose End process, close the Task Manager window)
-double-click on the Adobe Flash installer you just downloaded to install/update Adobe Flash
-when the Flash installation is complete, start Firefox, and test the Flash installation here: http://kb.adobe.com/selfservice/viewContent.do?externalId=tn_15507&sliceId=1
NOTE: On Vista and Windows 7 you may need to run the plugin installer as Administrator by starting the installer via the right-click context menu if you do not get an UAC prompt to ask for permission to continue (i.e nothing seems to happen). See this: http://vistasupport.mvps.org/run_as_administrator.htm
NOTE for IE: Firefox and most other browsers use a Plugin. IE uses an ActiveX version of Flash. To install/update the IE ActiveX Adobe Flash Player, same instructions as above, except use IE to download the ActiveX Flash installer.
If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.
You can use "Disable all add-ons" on the Troubleshoot Firefox issues using Safe Mode start window to disable all extensions.
You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")