Tracking cookies and MRU list generally aren't harmful... Tracking cookies are used by websites for a few things, to keep you logged in on some websites, (Im pretty sure sythe uses tracking cookies to keep you logged in, if it doesn't, rhq does to remember names you put in calcs), but they can also be used to track what your doing and such... They usually have an expire date on them but just to be on the safe side you can get rid of them once every week by going to Tools>Clear Private Data >Cookies > Ok in FireFox. MRU list is just most recently used list... Can possibly handy-cap your system if it builds up but again its usually not much of a problem.