Firefox – How tonstall Flash Player 10 without Adobe Download Manager

How-To-Geek had an article about installing Flash 9 in FireFox manually. Follow the steps to install into your profile not the All Users profile, as that will probably be locked down by the Corporate IT Department. I haven't tested it with Flash 10 myself.

I would warn however that your IT department may well frown upon installing Flash 10 this way. Depending on the organisations policies, manually installing software could be breaking the companies Acceptable Use Policy.

It seems to work in Chrome...

That is because Chrome already includes Flash plugin. It doesn't take into account that np32dsw.dll file you put in the plugins folder.

I just want to be able to play Youtube videos in Opera 12.

A lot of Youtube videos are play-able in the new HTML5 player. Activate it by going to http://youtube.com/html5 and see if videos play. You don't need Flash Player.