Do they just not play, or do you get an "unhappy plugin" crash message?

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

orange Firefox button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog that opens, you can remove your YouTube cookies. You might also want to change the site name in the box at the top of the dialog and remove your Google.com cookies if you have linked your logins.

Then try reloading the page. Does that help?

Does Flash work normally on other sites (e.g., Adobe's test page, Yahoo)?

If you have an ad-blocking extension installed, some users have reported needing to make an exception for YouTube.

Also, make sure all recorders/downloaders that interact with Flash media are as up-to-date as possible, or disable them.