If you let us know the sites concerned we maybe able to answer. There are often problems with using Flash Player. Firefox itself has a bug fix about to be included in a new point Release Fx45.0.1 but the advice may just be aimed at all Firefox version.

Have you got specific examples from public sites where Firefox shows big problems but Chrome and IE are good ?

http://videomega.tv/?ref= AJWOLaJQKbbKQJaLOWJA

Link broken by a moderator. The link redirects to various places including a scamsite

Thank you very much for those screenshots and the link. The short and simple answer is some websites are not coded well for Firefox. So when those sites are visited in Firefox, the performance is not great.

Now there aren't many video sites where I've seen this warning of "Do not use Firefox". But you are right. They do exist. But if they were more popular, more people would complain and the site would be fixed by the owners to make it have better performance in Firefox. But some sites think this is too much work and instead prefer to tell users to use another browser.

The site you linked to did indeed give me a warning not to use Firefox. It also on use redirected to advertisments and a scamsite video from a scamsite.

That particular site may be one to avoid.