Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

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 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")

Well, this is kinda a generic solution of yours, cor-el. As I expected, disabling addons and entering safe mode doesn't solve the problem.

This is a more or less wild guess: it seems the problem is with the couple newest Firefox + newest Flash update. I don't know who causes the problem, but the searches I made seems to also refer to Flash. As I have updated both recently, I cannot isolate one or another right now.

In some web page, it seems that all of those who provide Flash content should make a change to their crossdomain.xml files. As I'm just the client, I don't see a workaround right now.

My workaround right now: installed IE View Lite extension, and open these pages in Internet Explorer or whatever browser might work as expected.

Will be more than happy if there's any available workaround for Firefox itself.

When will those flash content pages finally reach the HTML5 nirvana? :-(