I can't uncheck 'Disable browser plugins'

I'd like to use a site which uses flash, but in the preferences/security settings i can't uncheck the 'disable browser plugins' line. When i uncheck it and hit ok, than i go back, it is checked again. I can uncheck the other lines. How could i make the flash work in tor browser?

Don't use Flash, use HTML5:

See bottom of this page:

https://www.youtube.com/html5

You may have to enable cookies and lighten up on some security settings in No-script and maybe elsewhere if you've tightened them.

Flash is proprietary and AFAIK doesn't work over Tor sessions. There are known exploits targeting Flash users in Tor sessions.